Donald Trump wrapped up back-to-back meetings with Republican lawmakers Thursday on Capitol Hill -- as he tried to rally and unify members of his own party ahead of the national convention in Cleveland.

The presumptive Republican presidential nominee first met with 200 House lawmakers at the Republican National Committee building in D.C.

Trump's message was, "We all need to stick together. Things will all work out in November," according to Rep. Ken Calvert of California.

Rep. Tom Price, R-Ga., told Fox News that Trump gave a “unifying speech” and that the presumptive Republican presidential nominee also spoke about the Supreme Court and the economy.

Rep. Trent Franks, R-Ariz., a Trump critic, told reporters that he will be a "vociferous supporter" of Trump because Franks is convinced that Democrat Hillary Clinton will "obliterate the Supreme Court, undermine this Constitution, and decimate this republic for a generation."

During his meeting with senators, Trump defended himself against some of his harshest critics.

Addressing Nebraska GOP Sen. Ben Sasse, he said, "Surely, you don't want Clinton." Sasse's spokesman, in a statement, said the senator considers the two presidential choices as a "dumpster fire," adding that "nothing has changed."

In one testy exchange, Trump recognized Sen. Jeff Flake of Arizona as a lawmaker critical of him. 

Flake referenced Trump's criticism last year of his colleague, Sen. John McCain, who was captured and spent 5 1/2 years in a Vietnamese prison.

Flake said he wanted to talk to Trump about those statements. The exchange, first reported by The Washington Post, left Flake unwilling to back the nominee.

"My position remains, I want to support the nomination. I really do. I just can't support him given the things that he's said," Flake told reporters later.

Sen. Mark Kirk of Illinois wasn't in Thursday's meeting, during which Trump supposedly called him a loser. Asked if Trump could win his home state in November, Kirk said, "I don't think so."

"We haven't seen a personality like his too much in the Midwest. Eastern, privileged, wealthy bully," Kirk said later. "Our bullies are made of better stuff in Illinois."

rump defended himself against some of his harshest Senate critics. Addressing Nebraska GOP Sen. Ben Sasse, he said, "Surely, you don't want Clinton." Sasse's spokesman, in a statement, said the senator considers the two presidential choices as a "Dumpster fire," adding that "nothing has changed."

In one testy exchange, Trump recognized Sen. Jeff Flake of Arizona as a lawmaker critical of him. Flake referenced Trump's criticism last year of his colleague, Sen. John McCain, who was captured and spent 5 1/2 years in a Vietnamese prison.

Flake said he wanted to talk to Trump about those statements. The exchange, first reported by The Washington Post, left Flake unwilling to back the nominee.

"My position remains, I want to support the nomination. I really do. I just can't support him given the things that he's said," Flake told reporters later.

Sen. Mark Kirk of Illinois wasn't in Thursday's meeting, during which Trump supposedly called him a loser. Asked if Trump could win his home state in November, Kirk said, "I don't think so."

"We haven't seen a personality like his too much in the Midwest. Eastern, privileged, wealthy bully," Kirk said later. "Our bullies are made of better stuff in Illinois."

A number of leading Republicans, including some in Congress who are facing tough re-election challenges in November, skipped the meeting with Trump. 

Sen. Kelly Ayotte of New Hampshire, who is facing a tight election race herself, told reporters she had to attend a Senate Armed Services Committee hearing at the same time as Trump's appearance.

Rep. Mario Diaz-Balart of Florida said he needed to check his calendar.

Sen. Marco Rubio of Florida said he was scheduled to preside over the Senate, but that others would benefit more anyway from seeing Trump.

"Obviously I'm very familiar with Donald and his positions; I just came off an 11-month campaign where he was one of my opponents," said Rubio, a former Trump rival who was often mocked by Trump as "Little Marco." “So some of the other folks perhaps wanted to spend more time learning more about his positions."

Another Trump rival, Texas Sen. Ted Cruz told reporters, "Donald asked me to speak at the Republican Convention and I told him I'd be happy to do so."  

He added, "There was no discussion of any endorsement." 

Thursday's meetings came just a few days after two potential vice presidential picks -- Sens. Bob Corker of Tennessee and Joni Ernst of Iowa -- indicated that they weren't interested in running on the same ticket as Trump.

One Republican who strongly backs Trump, Rep. Lou Barletta of Pennsylvania, said he was looking forward to his colleagues seeing a different side of Trump -- "a serious, business-like, boardroom Donald Trump -- not so much what they have been seeing on TV."

Trump's appearance came on the heels of a fiery speech in which he defended his use of a Star of David symbol in a retweet, an image that House Speaker Paul Ryan, R-Wis., and others have criticized.

Instead of focusing on Clinton during his remarks Wednesday in Cincinnati, as Republican leaders would have liked, Trump mixed his attacks on the presumptive Democratic nominee with a defense of the tweet, as well as earlier remarks touting former Iraqi dictator Saddam Hussein as a killer of terrorists. Trump argues the star in his tweet was a regular star that a sheriff might use.

Senate Majority Leader Mitch McConnell, R-Ky., who was quick to endorse Trump but has criticized him for going off-script and lagging in fundraising, said he looks forward to "a frank exchange."

"All of us are anxious to win the presidential election," said McConnell, who recently has said Trump's campaign is improving. "I think the one thing we agree on unanimously is four more years just like the last eight is not a good place for the American people."

The Trump meetings took place at the political headquarters of the Republican National Committee and the National Republican Senatorial Committee, within blocks of the Capitol. Trump planned to meet first with House members.

Ensuring an overheated atmosphere on Capitol Hill, FBI Director James Comey was to testify before a House committee at the same time as the Senate meeting with Trump. Comey was summoned by House Republicans who are irate over his recommendation that criminal charges not be brought against Clinton over her handling of classified emails during her tenure as secretary of state.

Democrats are moving to find political advantage from Trump's appearance. The Democratic Congressional Campaign Committee, which tries to elect Democrats to the House, released a new nationwide ad campaign Thursday seeking to link Republicans to Trump.

The ads, focused in the districts of 10 vulnerable lawmakers, will be running on cable networks; one of them, "Sidekick," likens Trump to a schoolyard bully and congressional Republicans to the bully's sidekicks and asks: "Shouldn't they really be standing up to the bully?"

Fox News' Chad Pergram, Kara Rowland and The Associated Press contributed to this report.