But Mr. Trump was not moved. “That’s not going to happen,” he said at different points during the evening, shrugging off the grim prognoses, according to multiple officials briefed on the conversation.
The disconnect between the president — a political novice whose confidence in his instincts was grandly rewarded in 2016 — and more traditional party leaders demonstrates the depth of the Republicans’ challenges in what is likely to be a punishing campaign year.
Mr. Trump is as impulsive as ever, fixated on personal loyalty, cultivating a winner’s image and privately prodding Republican candidates to demonstrate their affection for him — while complaining bitterly when he campaigns for those who lose. His preoccupation with the ongoing Russia investigation adds to the unpredictability, spurring Mr. Trump to fume aloud in ways that divide the G.O.P. and raising the prospect of legal confrontations amid the campaign. And despite projecting confidence, he polls nearly all those who enter the Oval Office about how they view the climate of the midterms.
According to advisers, the president plans to hold a fund-raiser a week in the months to come and hopes to schedule regular rallies with candidates starting this summer. But there is not yet any coordinated effort about where to deploy Mr. Trump, and there are divisions within his ever-fractious circle of advisers about how to approach the elections.
Among his close associates, a debate is raging about whether to focus on House races that could earn the president chits with Republican lawmakers who might ultimately vote on impeachment, or to dig in to defend the party’s tenuous Senate majority.
“We need to be unified, and I know this is a frustrating business that we’re involved in, but rather than having circular firing squads, we need to be shooting outward,” Senator John Cornyn of Texas, the second-ranking Senate Republican, said of the White House.
Nearly every modern president has lost seats in his first midterm election, and Bill Clinton saw both the House and the Senate fall to Republicans in 1994. But given Mr. Trump’s polarizing administration, the results this fall are likely to hinge more than ever on the man in the White House.
Anger toward Mr. Trump has become a crucial motivating tool for Democrats. Already, Republicans have spent millions on House special elections in strongly conservative areas of Pennsylvania and Arizona, losing one seat and retaining the other by a relatively narrow margin.
At the same time, Republican leaders believe he is an essential force for savaging Senate Democrats and turning out voters on the right.
Yet congressional leaders remain deeply frustrated about Mr. Trump’s improvisational pronouncements. At the White House dinner, Mr. McConnell raised one such policy and expressed hope that Mr. Trump could resolve the matter of his proposed tariffs, which have instilled deep worry among farm-state Republicans.
“If we can get trade resolved that would be exceptionally important,” Senator James Lankford of Oklahoma said when asked in an interview how the president could help in the midterms.
Other Republican lawmakers have begun pleading with the president to be disciplined and hold up the growing economy and sweeping tax overhaul they passed in December.
“He’s always defied political convention, but this is a political convention I think that we should adhere to,” said Senator Bill Cassidy of Louisiana, “which is to focus on that which is important to people, which is their wallet.”
When House Speaker Paul D. Ryan hosted a meeting of major Republican donors in Austin, Tex., this month, the head of the Congressional Leadership Fund, the primary House G.O.P. super PAC, delivered a presentation with a plea that the party “must sell the benefits of the Tax Cuts and Jobs Act” to retain the House.
Mr. Trump, though, has little appetite to carry a singular tax-cuts-and-the-economy argument and is grousing about what he sees as uninspired messaging by congressional leaders like Mr. Ryan.
Appearing in West Virginia this month at an event meant to showcase the party’s tax agenda, Mr. Trump discarded his prepared remarks — even describing them as “boring” — and turned to more incendiary issues like immigration.
Eric Beach, a Republican strategist who leads a pro-Trump political committee, Great America PAC, said Mr. Trump was rightly suspicious of the political formula favored by conventional Republican leaders like Mr. Ryan and Mr. McConnell.
“He doesn’t think that’s how you win elections because that’s not how he won his election,” Mr. Beach said. “He knows and understands that the core issues of today are illegal immigration — including building the wall — and trade inequity.”
Congressional leaders have left little doubt in private that they see Mr. Trump as a political millstone for many of the party’s candidates. In recent weeks, Mr. McConnell has confided to associates that Republicans may lose the Senate because of the anti-Trump energy on the left.
And at Mr. Ryan’s retreat, a Republican pollster, Kristen Soltis Anderson, identified Mr. Trump as a major source of the party’s woes, according to multiple attendees. Ms. Anderson noted that his job approval was markedly weaker than past presidents, including President Barack Obama in the months before Democrats lost 63 House seats in the 2010 elections.
Mr. Trump, for his part, has complained to associates about having been deployed to campaign for relatively weak Republicans like Roy S. Moore, who lost last year’s Senate race in Alabama, and Rick Saccone, who lost the special House election in Pennsylvania last month.
He has taken the losses personally, particularly in Alabama, because the vacancy there was a result of his decision to make Jeff Sessions attorney general, an appointment he has since regretted. Mr. Trump has subsequently blamed others in the party for thrusting him into episodes of humiliating defeat.
The scars from those races have made Mr. Trump reluctant to weigh in on the race that Senate Republicans most want his imprint on right now: the contest to replace Senator Thad Cochran of Mississippi, who resigned this month.
The president met this month with Senator Cindy Hyde-Smith, the Republican appointee there and the favorite of the party establishment. Reflecting his fixation on personal loyalty, Mr. Trump quizzed Ms. Hyde-Smith on whether she had supported another candidate for president in 2016 before endorsing him. When Ms. Hyde-Smith said she had not, the president exclaimed that he needed more supporters like her in Washington, people briefed on the meeting said.
But his staff pointedly told her not to request the president’s endorsement at the meeting. White House officials have created a series of fund-raising and organizational benchmarks that they want to see the new senator reach before they make a decision — a sign of how wary they are of entangling a president sensitive to political setbacks in elections that Republicans are not guaranteed to win.
Despite the lingering disputes with congressional Republicans, White House officials say the president is eager to return to the campaign trail.
Although some Republicans in competitive states may not want to appear with Mr. Trump — Gov. Scott Walker of Wisconsin, for example, has told associates he is unlikely to campaign with the president — there is no lack of lawmakers eager for his help.
Representative Lee M. Zeldin, Republican of New York, said he would welcome Mr. Trump on the trail anytime.
“I would expect the president and vice president to be in congressional districts all across the country,” Mr. Zeldin said. “I’ve gotten nothing but positive feedback on the desire of the president’s team to be as helpful as possible.”
What has stunned Republican veterans outside the White House is how, even 15 months into his presidency, Mr. Trump still lacks any unified political organization.
John F. Kelly, the White House chief of staff and a retired Marine general, has scant political acumen. And while the White House political staff has sought to bring a measure of order, curbing some of the president’s knee-jerk endorsement tendencies, Mr. Trump does not necessarily view them as his primary political counselors.
This vacuum has, as is often the case with this White House, triggered fierce internecine scrapping among those vying for Mr. Trump’s ear.
The president’s announcement that Brad Parscale, his 2016 digital guru, would manage his 2020 re-election campaign caught many of his most senior advisers by surprise, according to multiple Republicans. And the hasty decision immediately raised suspicions it was part of a power play by Jared Kushner, Mr. Trump’s son-in-law, to isolate Corey Lewandowski, the president’s 2016 campaign manager and occasional adviser.
Mr. Parscale has rankled Trump advisers by giving the president a perpetually rosy assessment of his poll numbers. He often tells Mr. Trump his numbers have “never been higher,” according to two advisers.
Mr. Parscale has also irritated some Trump officials by attempting to take over the political portfolio, with his scheduling of meetings to devise an as-yet-unformed midterm strategy getting back to other factions.
But his ascension marks only the newest power center in Mr. Trump’s political orbit: There is his White House staff, his vice president, the Republican National Committee, his family, his campaign alumni, his super PAC, his congressional allies, his conservative media friends and now his re-election team.
All are expected to want a voice in Republican strategy for Mr. Trump in the midterms, adding only more chaos, as one White House official phrased it, to an already unruly presidency.