In a meeting about  party unity,  Donald Trump had trouble holding his tongue (no surprise there) toward Senate Republicans who haven t always had the most flattering things to say about the GOP frontrunner. He threatened three of them with making sure none are reelected.Trump is famously very thin-skinned. Whenever he feels he s been wronged, which is basically all the time, or whenever someone fails to fawn all over him, he retaliates. He often takes to social media for retaliation, or the campaign trail, but this time, he s threatening more and today s biggest victim if Sen. Jeff Flake (R-Arizona).Flake isn t shy about saying that he thinks Trump would be very bad for the country. When he (Trump) refers to a judge born in Indiana as a Mexican in a pejorative way, when he calls those crossing the border rapists or when he goes after McCain (who Trump accused of being a coward because he was captured and held prisoner during the Vietnam War), somebody has to stand up and say,  This is not the Republican Party,    Flake added.  Because we ll have elections beyond this one, too, and we can t associate ourselves with those kinds of comments and be a serious national party. Source: Arizona CentralTrump responded by calling Flake  very critical of me. Flake, defending McCain, said,  yes, I m the other senator from Arizona   the one who didn t get captured   and I want to talk to you about statements like that.  Flake also insisted that he will not get behind Trump as a candidate. Trump then said that Flake will lose his election. Perhaps Trump should have uncharacteristically done his homework. Flake isn t running for reelection for two more years.The other victims of the unity meeting were Senator Mark Kirk (R-Illinois), who withdrew his endorsement of Trump and Senator Ben Sasse (R-Nebraska), who is advocating for a third-party candidate.Characterizing Kirk as a loser, Trump vowed that he would carry Illinois in the general election even though the state traditionally has been solidly Democratic in presidential contests. Kirk did not attend the meeting with Trump.Source: Washington PostTrump was more cordial to Sasse, who is still not on the billionaire s side. Senator Sasse went to today s meeting ready to listen. Senator Sasse introduced himself to Mr. Trump, and the two had a gracious exchange,  said James Wegmann, the senator s spokesman.  Mr. Sasse continues to believe that our country is in a bad place and, with these two candidates, this election remains a dumpster fire. Nothing has changed. Still, Republicans are by and large falling in line with the candidate. He was greeted by applause in the House, but that doesn t mean everyone is with him. Rep. Charlie Dent (R-Pennsylvania), says he still needs to be persuaded, so the door is still open.Rep. Mark Sanford (R-South Carolina), who s most famous for skipping out on his job and lying to his constituents while he had a tryst with his Argentinian girlfriend, had concerns that Trump didn t know enough about the Constitution. Some were concerned that Trump has praised Saddam Hussein. Others complained of the infamous Star of David tweet that Trump insists was a  sheriff s star.  In other words, there were many members of the room who had serious issues with Trump. Come November, though, you can bet they ll all be voting for him.Featured image via Drew Angerer at Getty Images.