WASHINGTON — Representative Kevin Cramer, Republican of North Dakota, said on Thursday that he would not challenge the Democratic incumbent senator, Heidi Heitkamp, in this year’s midterm elections. It was just the latest indication of the difficulty Republicans are having recruiting candidates in what had looked to be a highly favorable climate in the wake of President Trump’s inauguration a year ago.
Ms. Heitkamp is among ten Democrats seeking re-election in states won by Mr. Trump in his race against Hillary Clinton in 2016. Mr. Cramer had been seen as a top-tier challenger in a state the president carried by 36 points. He made his announcement during a North Dakota radio program.
“This is shaping up to be a tough cycle for Republicans across the board,” said Jennifer Duffy, a senior editor for the Cook Political Report who specializes in Senate races. “It only makes sense that recruiting is going to be difficult. For someone like Cramer, it doesn’t make a lot of sense to give up a safe seat for a difficult race in a lousy environment.”
That assessment is an abrupt reversal of how most analysts saw the midterm landscape shaping up a year ago.
Democrats are defending 24 seats, including those in the states where Mr. Trump won, compared to eight for Republicans. But the president’s chaotic first term has led to the lowest approval ratings for any first-term president at this time in his tenure in the history of polling, and he is now seen as a drag for many candidates. Two Independent senators, Bernie Sanders of Vermont and Angus King of Maine, are also up for re-election.
The environment also became hostile for Republicans who clashed with the president, and two of them, Senators Jeff Flake of Arizona and Bob Corker of Tennessee, surprisingly announced they would not seek another term. Those states are now rated as tossups by the Cook Political Report.
Mr. Cramer, who was at the White House last week as part of Mr. Trump’s effort to recruit him to run, is also not the only strong challenger to back out of the race. In Ohio, the state treasurer, Josh Mandel, recently announced that he would not run for the Senate in the race to oust the incumbent Sherrod Brown, a Democrat. But Republicans also gained a strong candidate on Thursday when Representative Jim Renacci said he would join the race.
Still, Republicans are expected to have highly credible challengers in states where Democrats are more at risk, including Missouri, where the states’s attorney general, Josh Hawley, is regarded as the top potential challenger to Senator Claire McCaskill; and Indiana, where three Republicans, including two House members, are competing with a wealthy business executive for the chance to take on Senator Joe Donnelly.
Ms. Heitkamp could still face a difficult contest against a candidate like Tom Campbell, a state senator and agribusiness executive. “I feel good about North Dakota,” Senator Cory Gardner, Republican of Colorado and chairman of the National Republican Senate Committee, told reporters on Thursday.
In the Senate, Republicans hold a slender 51-49 majority. Because Democrats have so many seats to defend in states won by Mr. Trump, Republicans were confident of maintaining control. But the unexpected retirements and Mr. Trump’s low approval ratings — even with an improving economy — are now reshaping political calculations in both parties.
Democrats also are more positive about their chances of retaking control of the House of Representatives thanks to a surge in Republican retirements that were at least in part a recognition of the drag that the president might be on the ticket.
On Wednesday, Representative Darrell Issa, Republican of California, became the 31st Republican to announce they were retiring or leaving the House. Mr. Issa represents the kind of largely suburban districts where Democrats see their greatest opportunity for gains.
Democrats would need a net gain for 24 seats to win control of the House.