As Donald Trump prepares to meet with top Republicans in Washington, House and Senate lawmakers are grappling with the new reality that the controversial New York businessman is the GOP's presumptive nominee.

Senate Majority Leader Mitch McConnell, R-Kentucky, who will meet with Trump on Thursday along with his leadership team, has offered a muted endorsement of Trump’s campaign, telling reporters he won “the old-fashioned way.”

But he did not say whether fellow Republican senators should back the nominee, a position shared by Trump’s onetime opponent Sen. Marco Rubio, R-Florida.

“Everybody will have to make their own decision,” Rubio told ABC News in a brief interview at the Capitol today.

Asked by ABC News later in the day if he would vote for Trump, Rubio did not directly respond.

“Sorry guys, I’m late for my 2 o’clock,” he said.

In his return to Capitol Hill Tuesday, Sen. Ted Cruz, R-Texas, refused to say if he’d endorse Trump or tell his supporters to back the campaign, but said he had "no interest" in a third-party run.

Sen. Bob Corker, R-Tennessee, the Senate Committee on Foreign Relations chairman who praised Trump’s recent foreign policy address, said he plans to support the GOP nominee, and has offered to help the campaign on foreign policy.

Asked about his colleagues who continue to reject Trump, Corker said, “Let’s chill.”

“I haven’t understood people’s rush to get to a certain place,” he said, adding that the campaign is evolving.

Corker said he has “no reason to believe” he’s being considered for vice president by Trump, but did not fully rule himself out.

Trump will return to Washington Thursday for the first time as the GOP’s presumptive nominee, in an effort to patch up strained relations with party elders after a contentious primary.

Speaker Paul Ryan, R-Wisconsin, who has withheld his endorsement for Trump over concerns about his campaign, tamped down expectations for his meeting with Trump in a series of interviews Tuesday.

“We need to start talking about how we unify the Republican Party,” Ryan told the Wall Street Journal. “We shouldn’t just pretend our party is unified when it’s not.”

ABC News’ Ali Dukakis contributed to this report.