Unlike previous presidents, Donald Trump has made many endorsements in primary elections. How much impact did they have?
Previous presidents rarely endorsed in contested primary elections. This may have been because they wanted to be seen as 'above petty politics'. Or perhaps they didn't want to risk their endorsee losing, which would weaken their political standing.
However, presidents have ways to intervene in primaries besides making endorsements. They can help to raise money, round up endorsements by other party leaders, or instruct the party apparatus to support a candidate. Previous presidents have relied more heavily on subtle means like these. President Trump has had less control of the party establishment, but a stronger connection to the grassroots. This likely explains his heavier reliance on endorsements to influence the Republican party.
Ballotpedia has a convenient list of Trump's endorsements, from which the list in this article is adapted.
This article is concerned with endorsements of a single candidate in Republican primaries (including jungle primaries). I eliminated WI-SC (nonpartisan) and the multi-candidate endorsements in WV-Senate and LA-Governor. I added NB-Senate, which was misplaced on Ballotpedia's list. I am not interested in general election endorsements, where the impact of a partisan endorser is likely to be negligible.
My methodology is to look at public polling and election results before and after Trump endorsed, and see whether there was a change. If Trump's endorsement had an impact, the endorsee's number should improve, whether immediately or over time.
There are various limitations to this methodology. Polling has both a theoretical margin of error, and practical difficulties finding a representative sample. Nonetheless, it is still useful for observing trends.
We should also note that the effect of endorsement will vary from race to race. Primary electorates in some districts are more friendly to Trump than others. Endorsements generally have more impact when candidates are less known, which generally includes open seats.
RACES WITH INCUMBENTS
I separated the races with R incumbents into endorsements of incumbents and endorsements against incumbents. The latter category was split into controversial and uncontroversial incumbents. The latter distinction is somewhat subjective, but it attempts to separate incumbents who were in real danger of losing from those who faced hopeless, underfunded challengers. Endorsing a bunch of incumbents who would win anyways is an easy way for an endorser to increase the winning percentage.
WIN AZ-4 (2020) Paul Gosar
WIN MS-3 (2020) Michael Guest
WIN NY-27 (2020) Christopher Jacobs
WIN WV-Governor (2020) Jim Justice
WIN AZ-Senate (2020) Martha McSally
WIN MO-Governor (2020) Mike Parson
WIN NB-Senate (2020) Ben Sasse
WIN NJ-2 (2020) Jeff Van Drew
WIN CA-8 (2018) Paul Cook
WIN NY-11 (2018) Dan Donovan
WIN AZ-Governor (2018) Doug Ducey
WIN TN-8 (2018) David Kustoff
WIN TX-Ag Comissioner (2018) Sid Miller
I didn't check for polls of the house races, since it is unlikely there were any for most of them. There don't seem to have been any polls for the AZ-Senate, and there was only one in Nebraska. For AZ-Governor, there was only one primary poll. For WV-Governor, the polls don't seem to have changed significantly after Trump's endorsement of Justice.
For MO-Governor, Parson did better in the final result than in the two polls before Trump's endorsement, but those polls included a potential candidate (former governor Eric Greitens) who didn't end up running. It is difficult to say whether Trump's endorsement had any effect.
WIN TX-12 (2020) Kay Granger
WIN UT-AG (2020) Sean Reyes
LOSS VA-5 (2020) Denver Riggleman
LOSS CO-3 (2020) Scott Tipton
WIN SC-Governor (2018) Henry McMaster
WIN AL-2 (2018) Martha Roby
LOSS AL-Senate (2017) Luther Strange
There don't appear to have been any public polls for TX-12, UT-AG, VA-5, CO-3, or AL-2. The VA-5 loss was at a convention.
For SC-Governor, a poll shortly before Trump's endorsement of Henry McMaster had him at 33% and 50% undecided. About two months later, a poll had him at 51%. McMaster eventually won the runoff by 7%.
In AL-2, Martha Roby got 39% in the primary, but after Trump's endorsement got 68% in the runoff. Trump may have helped get voters who supported the other candidates to back Roby.
In AL-Senate, a poll two weeks before Trump's endorsement of Strange had Roy Moore leading by 2%. The first poll after Trump's endorsement had Moore leading by 19%, and he maintained a roughly 10% average lead for the rest of the race, and won by that amount. Trump's endorsement didn't help here.
Challenges Against Incumbents:
WIN KS-Governor (2018) Kris Kobach
WIN SC-1 (2018) Katie Arrington
For KS-Governor, polls were about even between Kobach and Jeff Colyer. The final poll showed Colyer leading by 2%. After Trump's endorsement, Kobach won by 0.1%. Trump's endorsement doesn't seem to have had much impact here.
In SC-1, a poll a month before the election had Mark Sanford leading by 1%. Trump's endorsement came the day of the primary. Sanford eventually lost by 4%. If Trump's endorsement had any effect, it was small.
OPEN SEAT PRIMARIES
WIN OH-15 (2021) Mike Carey
LOSS NC-11 (2020) Lynda Bennett
WIN MN-7 (2020) Michelle Fischbach
WIN TX-23 (2020) Tony Gonzales
WIN TN-Senate (2020) Bill Hagerty
WIN TX-7 (2020) Wesley Hunt
WIN TX-13 (2020) Ronny Jackson
WIN NH-Senate (2020) Corky Messner
WIN NH-1 (2020) Matt Mowers
WIN TX-11 (2020) August Pfluger
WIN MT-AL (2020) Matt Rosendale
WIN PA-7 (2020) Lisa Scheller
WIN AL-Senate (2020) Tommy Tuberville
WIN CA-Governor (2018) John Cox
WIN FL-Governor (2018) Ron DeSantis
LOSS WY-Governor (2018) Foster Friess
WIN MI-Senate (2018) John James
WIN GA-Governor (2018) Brian Kemp
WIN UT-Senate (2018) Mitt Romney
There don't appear to have been any public polls for NC-11, MN-7, TX-7, or PA-7.
In OH-15, a poll after Trump's endorsement of Carey found him leading with 20%, and he won with 36%. It is reasonable to think that Trump's endorsement provided a big boost, but this can't be proved with polling data.
In TX-23, Tony Gonzales led the primary by 5% before Trump's endorsement, and won the runoff by 0.2%, so Trump's endorsement doesn't seem to have helped here.
In TN-Senate, Trump endorsed Bill Hagerty before he was even in the race. This froze out some elected officials (like Rep. Mark Green) who were considering running. Hagerty led from the beginning, and won the primary comfortably. Trump's endorsement made a huge difference here.
In TX-13, Ronny Jackson got only 20% in the primary, shortly after Trump's endorsement, but grew to 56% in the runoff. Perhaps Trump's endorsement helped over time.
In NH-Senate, Corky Messner gained steadily after Trump's endorsement and won comfortably. The endorsement may have helped here.
In NH-1, Matt Mowers gained in the polls and won easily, so Trump's endorsement may have helped here.
In TX-11, there doesn't seems to have been any polling, but since August Pfluger won easily, it is reasonable to think Trump's endorsement helped signigicantly.
In MT-AL, there isn't enough evidence that the results changed after Trump's endorsement.
In AL-Senate, polling shifted substantially toward Tommy Tuberville after Trump's endorsement. Notably, Trump campaigned agressively against Jeff Sessions, unlike many other endorsements that were just positive tweets.
For CA-Governor, John Cox gained at most a couple points after Trump's endorsement in the jungle primary, so the endorsement didn't seem to help much here.
For MI-Senate, polling swung to John James after Trump's endorsement. Trump's endorsement seems to have helped here.
For FL-Governor, Adam Putnam led most early polls, but after Trump's endorsement, Ron DeSantis took the lead, and eventually won comfortably. Trump's endorsement certainly helped here.
For WY-Governor, Trump endorsed Foster Friess on the day of the primary, too late to have any impact on the race.
For GA-Governor, Brian Kemp had already taken a significant lead by the time Trump endorsed him. He won by a larger margin, so Trump's endorsement may have helped to run up the score.
For UT-Senate, all polls occurred after Trump's endorsement. Mitt Romney was always a strong favorite here, so it likely didn't make much difference.
Trump's endorsements in open seat races were still somewhat cautious. Fischbach, Hunt, Rosendale, Scheller, Cox, Kemp, and Romney were all clear favorites before Trump endorsed them. Romney (and Ben Sasse) faced more pro-Trump challengers, and eventually voted for impeachment. Many of Trump's other endorsements were candidates who were co-leaders (DeSantis, James) or in fields of unknowns.
INFORMED VOTER POLLING
Another way of evaluating the impact of Trump endorsements is polls that ask voters about a race before and after informing them about his endorsement. One example is a poll of the GA governor primary by InsiderAdvantage.
GA-Governor (Trump endorsed David Purdue)
Before: Kemp 41, Purdue 21, Jones 11
After: Kemp 34, Purdue 34, Jones 10
Purdue increases by 13, and Kemp drops by 7.
Similar polls have been conducted by Club for Growth, which endorsed each of the candidates that Trump endorsed.
AL-Senate (Trump endorsed Mo Brooks)
Before: Brooks 55 Britt 12, Und 23
After: Brooks 72 Britt 13, Und 9
Brooks gained 17, mostly from undecideds.
NC-Senate (Trump endorsed Ted Budd)
Before: Budd 21, McCrory 45, Walker 13, Und 21
After: Budd 52, McCrory 28, Walker 8, Und 12
Budd gains 31, pulling from all the other options.
OH-16 (old)/OH-13 (new) (Trump endorsed Max Miller)
Before: Miller 39, Gonzalez 30, Und 31
After: Miller 69, Gonzalez 17, Und 14
Anthony Gonzalez later decided not to run for reelection.
Note that these shifts are the best case scenario, since not all voters will hear about Trump's endorsement, and these polls only provide this one additional piece of information for voters to consider.
Donald Trump's endorsements had essentially no impact on primary races with incumbents. In AL-Senate, the polls actually moved against the candidate he endorsed. There were two cases where Trump-endorsed challengers beat incumbents. However, Jeff Colyer had not been elected governor in his own right, and Kris Kobach was likely the favorite from the beginning. In SC-1, Mark Sanford only won the 2016 primary 56-44, so he had serious baggage prior to Trump's late endorsement.
Trump's endorsements had a more significant impact on open seat races. Bill Hagerty, Tommy Tuberville, August Pfluger, Mike Carey probably owe their victories to Trump. In other races, Trump seems to have helped less, and in some, he may not have helped at all.
The informed voter polls show that voters are more likely to support a candidate endorsed by Donald Trump--up to 30% of voters switched, in some cases. However, in a real campaign, voters base their votes on a combination of factors, which explains why such dramatic shifts have not been observed in actual campaigns.
Trump's endorsements remained somewhat cautious approach prior to 2021. Since losing the 2020 election, Trump has pursued a different strategy. He has made many more endorsements, including downballot races like state legislatures. He has also shown far less concern for electability, endorsing some candidates who seem to be clear underdogs in either primaries or generals. This will provide a clear test of his continuing influence on Republican primary elections.