Speaker Paul Ryan has backed away from his pledge to support whoever becomes the nominee, saying he's "not ready" to endorse Republican presidential candidate Donald Trump. Other GOP heavyweights, including the Bushes, are also not giving endorsements. (Jenny Starrs/The Washington Post)

National Republican leaders remained sharply divided Tuesday over the likely coronation of Donald Trump as the party’s standard-bearer, foreshadowing a lengthy battle in the two months leading up to the presidential nominating convention in Cleveland.

House Speaker Paul D. Ryan (R-Wis.) dug in for a protracted discussion with Trump after last week’s stunning declaration that he was “not there yet” on embracing his party’s likely presidential nominee. Ryan’s ambivalence raised the stakes on his meeting with the businessman slated for Thursday at the Republican National Committee headquarters on Capitol Hill.

“It’s going to take more than a week just to repair and unify this party,” Ryan said in an interview Tuesday with the Wall Street Journal’s Gerald F. Seib, broadcast online by Facebook. “If we just pretend we’re unified without actually unifying, then we’ll be at half-strength in the fall, and that won’t go well for us.”

Ryan’s statement came as many senior Senate Republicans called for a detente in the showdown between both the party’s establishment and conservative wings vs. Trump’s insurgent campaign, which effectively took over the party with a message that runs contrary to many bedrock conservative principles. But Ryan also clarified that no endorsement of Trump should be expected after Thursday’s get-together.

“I think the single most important thing is that Republicans and primary voters unify behind our party’s nominee and then make sure we defeat Secretary Clinton,” Sen. John Cornyn (Tex.), the No. 2 Republican leader, told reporters Tuesday. That refrain, defeating Hillary Clinton, the likely Democratic nominee, served as a rallying cry for most Republicans who have come on board Trump’s candidacy.

Donald Trump speaks during an interview with The Associated Press Tuesday in his office at Trump Tower in New York. (Mary Altaffer/AP)

The sharp Republican divisions were on display as Congress returned for its first full day in the Capitol after a 10-day break, after a recess beginning with Republicans leaving Washington girding for a contested convention in Cleveland and ending with Trump as their party’s standard-bearer. Republicans seemed of three minds as they faced the voters’ verdict: many supported, albeit reluctantly, the party’s presumptive nominee; a handful enthusiastically backed him; still others firmly reject his candidacy.

In an effort to bridge the divide, Trump will descend on Washington on Thursday for a series of meetings, beginning in the morning with Ryan and then his leadership team, and concluding later across Capitol Hill with Senate Majority Leader Mitch McConnell (Ky.) and his top lieutenants.

[Paul Ryan: GOP unity will “take more than a week”]

It was a day of mixed messages and a fatigue from lawmakers on Trump questions that are only likely to increase before November.

Ryan’s press office began Tuesday with a plea to the media to focus on other issues besides Trump and then sent the speaker into interviews with the Wall Street Journal and conservative talk radio specifically to talk about the presidential campaign.

Exasperated with the Trump phenomenon, Sen. Mark Kirk (Ill.), the most endangered Republican incumbent in the November elections, waved off reporters.

“We’re not doing any Trump questions,” Kirk said.

House Speaker Paul Ryan (R-Wis.) arrives on Capitol Hill Tuesday. He is expected to meet with Republican presidential candidate Donald Trump on Thursday. (Susan Walsh/AP)

The pro-Trump Republicans, hardly enthusiastic, tend to view the political neophyte as a cipher on most policy issues that can be molded in a more classic conservative image to run a more conventional general-election campaign.

Sen. Bob Corker (R-Tenn.), chairman of the Foreign Relations Committee, delivered an almost lackadaisical rebuke to the “Stop Trump” wing: “Chill. And let the campaign evolve a little bit, and see where the candidate ends up.”

Corker described Trump’s emerging worldview as similar to that of former president George H.W. Bush and his advisers, more reluctant to use troops and military force than his son’s administration, and he defended Trump’s criticism of NATO, which previously drew stern rebukes from many conservative national security experts.

Corker, a proud member of his state’s entrenched Republican establishment, wouldn’t even rule out discussion of him becoming Trump’s vice-presidential nominee at the Republican convention in July.

In his Tuesday interviews, Ryan avoided some of the sharp criticism he delivered of Trump last week, when he asked whether the businessman was actually conservative and questioned his campaign of “belittlement” toward women and minorities. He tried to decrease expectations for his Trump meeting and made clear there would be no endorsement after it, suggesting it would take longer to see if the party’s presumptive nominee would support the emerging conservative agenda that Ryan has been working on since becoming speaker last fall.

“We can’t fake it. We can’t pretend. We have to actually unify and do it,” Ryan said in the Journal interview.

[How Paul Ryan and Mitch McConnell took such different approaches to supporting Donald Trump]

Ryan’s position angered some Republicans, who suggested that Trump’s strong anti-immigration, anti-trade and more retrenched foreign policy views had won out over the more traditional conservatives opinions espoused by Ryan.

“Trump is not going to change his fundamental campaign themes. He believes what he says and the people have ratified it,” Sen. Jeff Sessions (Ala.), the first Republican senator to endorse Trump, told reporters, saying it was “a mistake” to hold out support. “A number of candidates took Speaker Ryan’s position, and they’re no longer heard of. They have disappeared.”

Those strong views have driven devout conservatives into a frenzy, unsure of how to handle the nomination of someone whose positions fall so far outside the historical Republican mainstream.

Sen. Lindsey O. Graham (R-S.C.), perhaps Trump’s staunchest GOP critic on Capitol Hill, reiterated his past opposition to ever voting for him. But Graham also rejected the idea from some conservatives of running an independent conservative as a third option to Trump or Clinton.

“I’ll probably write somebody in or just skip the presidential,” Graham said.

McConnell told reporters that Trump may be underestimated in his ability to win votes through his unconventional campaign style, adding there was no choice but to work with the party’s nominee. If Republicans refuse to get on the Trump train, McConnell said Clinton’s victory would lead to “four more years” of President Obama’s policies.

“I think most of my members believe he’s won the nomination the old-fashioned way — he got more votes than anybody else and we respect the voices of the Republican primary voters across the country,” McConnell said.

Karoun Demirjian, Ed O’Keefe and Kelsey Snell contributed to this report.