Donald Trump is the presumptive GOP presidential nominee now that his last remaining opponents Ted Cruz and John Kasich have dropped out. That fact doesn't sit well with many Republican politicians who don't support Trump as their party's main choice. 

Paul Ryan, House Speaker and chairman of the Republican National Convention, appeared on CNN Thursday, contradicting his earlier comments about supporting the eventual GOP nominee. "I'm just not ready to do that at this point," he said of supporting Trump. "I'm not there right now." He remained open to endorsing the party's candidate, if that person took a more unifying stance.

Trump responded to Ryan's comments in a statement, saying: "I am not ready to support Speaker Ryan's agenda. Perhaps in the future we can work together and come to an agreement about what is best for the American people. They have been treated so badly for so long that it is about time for politicians to put them first!"

Ryan isn't alone in his hesitation. In fact, several Republican politicians have said they would not back the billionaire businessman at all. 

Mitt Romney announced Thursday he won't attend the 2016 GOP Convention in Cleveland come July. Former presidents George W. Bush and George H. W. Bush, as well as former Gov Jeb Bush and Sen. John McCain have also said they will not attend, according to CNN. By skipping the convention, each former Republican presidential nominee sends a strong signal about their lack of support. 

A number of other politicians have also voiced their dissent. Nebraska Sen. Ben Sasse, Virginia Rep. Scott Rigell, Florida Rep. Carlos Curbelo, New York Rep. Richard Hanna, and Massachusetts Gov. Charlie Baker have each said they would not endorse Trump, according to MSNBC.

Sasse went so far as to pen a lengthy open letter about his disappointment in both Trump and Hillary Clinton. 

Trump's presumed Democratic opponent Clinton continues to beat him in projected polls. Perhaps that's why his latest tweet suggested Sen. Bernie Sanders enter the fray no matter the outcome of the Democratic nomination.