Rep. Kevin Cramer (R-N.D.), right, talks about being one of the first to endorse Donald Trump for president as Trump meets with North Dakota delegates to the Republican National Convention in May 2016. (Charles Rex Arbogast/Associated Press)

The Republican effort to enlist formidable candidates for the ­Senate suffered another setback Thursday when Rep. Kevin Cramer (R-N.D.) announced that he will not run despite President Trump's personal appeals for him to challenge Democratic Sen. Heidi Heitkamp in one of the year's top battlegrounds.

Cramer's decision to instead seek reelection in the House, which he announced in a radio interview, followed other recruiting letdowns for Trump and Senate Majority Leader Mitch McConnell (R-Ky.) in deeply conservative states where Republicans are hoping to unseat Democratic senators and pad their 51-49 majority.

"That's a disappointment. Yeah, I know Kevin — he would've been a great candidate," said Sen. John Thune (R-S.D.), one of McConnell's top lieutenants.

At the beginning of 2017, with Trump's inauguration, Republicans were optimistic about increasing their Senate majority. With Democrats defending more than three times as many seats as the GOP, Republicans saw opportunities to build — perhaps even reach the filibuster-proof number of 60 — and give themselves considerable leverage on future legislation.

But attracting top candidates in key Senate races has been an ongoing struggle for a party facing strong political head winds. A pair of Republican retirements — Jeff Flake in Arizona and Bob Corker in Tennessee — have opened the door for Democrats in GOP-held states. In Montana, Wisconsin, Michigan and now North Dakota — all states that Trump won and where Democratic senators face elections this year — top Republicans have passed on the Senate contest to pursue other opportunities.

Democrats will be looking to hold on to 26 seats, including in Minnesota after Al Franken stepped down amid sexual misconduct allegations, and the two independent seats in Vermont and Maine. Republicans have eight seats on the ballot in November after Democrat Doug Jones stunned the GOP with a win in ruby-red Alabama last month.

Cramer, who has been a staunch supporter of Trump and has a higher profile than most Republicans in his state — North Dakota has only one seat in the House, making its representative more visible than most — said family considerations led him to conclude that it was in his best interest to run for the House, not the Senate.

"It's far less intense than flying around the country for the next 10 months every weekend, going to Chicago and New York and, you know, cities far away to raise adequate funds to, you know, to run," he said on the radio program "What's on Your Mind?"

Cramer said he did not want to "diminish the impact" of Trump urging him to run. He added that "while he was very persuasive, he was also very understanding when we raised concerns."

Trump extensively courted Cramer for the race, even calling his wife to make his pitch. The two met on Jan. 2 in the Oval Office. By the end of 2017, White House officials had thought Cramer was going to run and were even predicting the date of his announcement. Senate Republican officials were also keen about getting him onboard.

The North Dakota Senate race is a top priority for Republicans this year. It is one of 10 states Senate Democrats are defending that Trump won. He carried the state by more than 36 percentage points in 2016.

Republican state Sen. Tom Campbell has announced his candidacy, but he is not as well known as Cramer. Other Republicans could be tempted to challenge him, triggering a potentially bruising primary.

Heitkamp was reluctant to talk about Cramer's decision. As she boarded a subway in the Capitol, she tried to brush aside questions. "I've issued a statement," she said. A campaign representative provided a statement from Heitkamp that made no mention of Cramer.

"It's telling that people don't want to take on that race — and for good reason. Because Heidi's been all about North Dakota," said Sen. Chris Van Hollen (Md.), chairman of the Democratic Senatorial Campaign Committee.

Cramer's decision adds to the list of Trump's personal recruiting missions that have proved unsuccessful.

The president tried to convince Sen. Orrin G. Hatch (R-Utah) to run for reelection — even organizing a trip to Utah last year largely to court him. Aides made sure Hatch was alone with Trump in certain moments and photo opportunities.

But Hatch announced earlier this month that he plans to retire, clearing the way for Mitt Romney, a frequent Trump critic, to launch a likely bid for his seat.

Trump's political director, Bill Stepien, frequently met with Ohio Treasurer Josh Mandel, who ended his Senate campaign last week, citing an unspecified health problem his wife is dealing with as the reason for his decision.

After Mandel ended his bid, the White House was successful in nudging Rep. James B. Renacci (R-Ohio) to run instead. Renacci met with Stepien on Wednesday at the White House and announced Thursday that he was ending his campaign for governor to run for the Senate.

Still, the sting of Cramer's decision could be felt in the hallways of the Capitol, where some Republicans were plainly deflated by the news.

"Very disappointing. He's a good congressman. He would have made a very good United States senator," said Sen. Mike Rounds (R-S.D.).