Let’s start with the kingpin of the Republican Party, the president himself. Trump endorsed 17 candidates in open Republican primaries this election cycle, and 15 of them won. That 88 percent win rate is the highest of any person or group we looked at. In early August, Trump tweeted, “As long as I campaign and/or support Senate and House candidates (within reason), they will win!” It was a bit of an exaggeration, but his success rate has certainly been high so far.
But the evidence is mixed on how much credit Trump deserves for actually driving those primary wins. On the plus side for Trump, in the GOP primary for Florida governor, state Agriculture Commissioner Adam Putnam led in most polls before June 22 — but then Trump tweeted his full endorsement of then-U.S. Rep. Ron DeSantis (who resigned from the House this month). DeSantis surged into the lead, and he topped practically every poll the rest of the way. On the other hand, in California’s gubernatorial primary, Republican John Cox polled about equally well both before and after receiving Trump’s endorsement. Cox averaged 16.5 percent support among voters of all parties — in California primaries, candidates of all parties appear on the same ballot5 — in four polls taken the month before Trump’s May 18 endorsement. Cox averaged 17.7 percent in three polls taken after the president’s endorsement.6 But the role Trump played in the Georgia gubernatorial primary is less clear-cut: We have some evidence that some, but not all, of Kemp’s rise was due to Trump. An Opinion Savvy poll that happened to be in the field when the president gave his endorsement found that Kemp was already outperforming his previous polling even before Trump tweeted.7 But the same poll also found that Kemp polled even better after Trump’s endorsement.