Republican presidential candidate Donald Trump speaks during an interview with The Associated Press in his office at Trump Tower, May 10, 2016, in New York.
Republican presidential candidate Donald Trump speaks during an interview with The Associated Press in his office at Trump Tower, May 10, 2016, in New York.

Billionaire real estate mogul Donald Trump is the presumptive U.S. Republican presidential nominee, but he continues to struggle to win support for his maverick candidacy from key Republican figures.

Two Republican presidential contenders Trump defeated in his months-long run to the top of the party's presidential field, Texas Senator Ted Cruz and Florida Senator Marco Rubio, returned to the halls of Congress this week but showed little enthusiasm for Trump.

Cruz, a conservative firebrand who called Trump "utterly amoral" before dropping out of the presidential contest a week ago, refused to say whether he would endorse his one-time opponent or encourage his supporters to back Trump in his likely contest with the Democratic front-runner in the November national election, former secretary of state Hillary Clinton.

"There will be plenty of time for voters to make the determination of who they're going to support," Cruz said. "It will be incumbent on the candidates in this race to make the case to the people that they will fight for them."

FILE - then-Republican presidential candidate Sen
FILE - then-Republican presidential candidate Sen. Marco Rubio, R-Fla., speaks during a campaign rally at Palm Beach Atlantic University in West Palm Beach, Fla.

Rubio vows to support GOP nominee

Rubio said he would adhere to his months-old pledge to support the Republican presidential nominee. Rubio, however, said he still has deep concerns about Trump's policies, that he is not a reliable conservative, and the way the brash, one-time television reality show host conducted his campaign.

"All of the policy differences I have with him remain," Rubio told NBC's Today show on Wednesday. "All of the reservations I have about his campaign remain. I clearly didn't want us to be in the position we are in today."

Rubio said he has "even more policy differences with [Clinton] and I'm even more scared about her being in control of the U.S. government and continuing the status quo of Barack Obama's policies."

FILE - Republican presidential candidate, former F
FILE - Republican presidential candidate, former Florida Gov. Jeb Bush visits a polling place at the Eastlan Baptist Church in Greenville, S.C.

Bush will abstain

Another of Trump's vanquished foes, former Florida governor Jeb Bush, says he will not vote for either Trump or Clinton. Bush's father and brother, the last two Republican presidents, George H.W. Bush and George W. Bush, have both declined to become involved in the 2016 campaign and are skipping July's Republican National Convention, where Trump will be formally nominated.

Numerous Republican lawmakers also say they plan to avoid the convention, although others say they plan to attend and support Trump's candidacy, in part because he has won nearly 11 million votes from Republican supporters in the state-by-state nominating contests that started in February and extend into early June.

Senate Majority Leader Mitch McConnell of Ky., wit
FILE - Senate Majority Leader Mitch McConnell of Ky., with, from left, Sens. John Barrasso, R-Wy., John Thune, R-S.D., and Senate Majority Whip John Cornyn, R-Texas, speaks to reporters on Capitol Hill in Washington, May 10, 2016.

Washington meetings

Trump heads to Washington Thursday for meetings with key Republican congressional leaders, including the party's top current elected official, House of Representatives Speaker Paul Ryan, who says he is "just not ready" to endorse Trump as the party's standard bearer.

"We can't fake that we're unified," Ryan said, adding that he needs to discuss Trump's underlying political philosophy with him.

Trump initially said he was "blindsided" by Ryan's refusal to endorse him and that it is possible the two may just "go our separate ways."

On Tuesday, however, Trump called Ryan "a very good man. We'll see what happens" at the meeting. "If we make a deal, that'll be great. If we don't, we'll trudge forward like I've been doing," he said.

Ryan is slated to chair the party's national convention and offered to drop that role if Trump, as the party's nominee, wanted him to do so; but, Trump said, "I'd love frankly for him to stay and be chairman."