House Speaker Paul Ryan, in a television interview on Thursday, said he’s not yet “ready” to support or endorse Donald Trump – marking the latest flare-up with the party as the presumptive Republican nominee quickly finds himself engaged in a two-front battle on the road to November.

Hillary Clinton’s juggernaut campaign operation already is hitting him hard, but the billionaire businessman still faces resistance from some Republican critics. While several former primary rivals have come around to his campaign, some GOP figures are outright opposing his nomination. Others, like Ryan, are equivocating.

Ryan, asked Thursday if he’d back Trump, said: “I'm just not ready to do that at this point.” 

Speaking on CNN, Ryan suggested he wants Trump to do more to unify the party first and confirmed he is not endorsing the candidate right now. 

At the same time, Ryan called Trump the presumptive nominee, saying, “I hope to” support him and “want to.”

Trump quickly fired back 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!" Trump said.

Ryan’s reluctance to offer his full-throated support, though, signifies the immensity of the task ahead for Trump in unifying the party, especially considering Ryan will serve as chairman of the GOP convention in Cleveland. Unless and until the tensions with fellow Republicans are settled, Trump will be in the historically unique position of parrying hits from his own party even after sealing the nomination – all while preparing to face the well-oiled Clinton machine.

Clinton’s campaign also has been eager to point out any rifts in the party, and quickly blasted out a message claiming Ryan had joined the “growing list of conservatives rebuking Trump.”

The Clinton camp also dropped two ads on Wednesday: one attacking Trump for past controversial statements, and the other using the words of his Republican opponents against him.

The second ad shows key party figures including 2012 Republican nominee Mitt Romney and former Florida Gov. Jeb Bush attacking Trump. Among the quotes, the video shows Bush saying “he needs therapy.” 

Despite calls from Republican National Committee chief Reince Priebus for Republicans to unite around the likely nominee, Ryan wasn’t the only senior figure voicing concerns.

Fox News confirmed Thursday that Romney – a vocal Trump critic -- has no plans to attend the convention, while Presidents George H.W. Bush and George W. Bush both have confirmed they will not attend either. A source close to former Senator and 1996 Republican nominee Bob Dole said he would “briefly” attend the convention but primarily to attend a lunch hosted by his law firm.

Other GOP figures have been more outspoken. Nebraska Sen. Ben Sasse, in an open letter “to majority America,” called for a third-party candidate. “With Clinton and Trump, the fix is in. Heads, they win; tails, you lose. Why are we confined to these two terrible options? This is America. If both choices stink, we reject them and go bigger. That’s what we do,” Sasse wrote.

Trump, though, has signaled he’ll move to unite the party and says some of the people who used to criticize him are now reaching out to him to get on board.

Several former opponents have endorsed him, including Ben Carson, who is now involved in Trump’s running-mate selection process.

Carson, speaking with PBS, predicted that the opposition from Republican will “melt away as people begin to consider the alternative.”

He added: “Now, there may be some Republicans who, you know, prefer a more progressive ideology, prefer, you know, more debt, prefer withdrawal from the world and allowing our enemies to multiply and destruction of our military. And those people probably should … move on.”

The opposition from Republican lawmakers all along has formed part of the broader “Never Trump” movement that has taken off on social media and among conservative media outlets.

Despite calls to unite behind Trump, the official #NeverTrump PAC tweeted its continued opposition Tuesday night, as Trump won the Indiana primary and knocked his remaining rivals out of the race.

Editor of the Weekly Standard Bill Kristol said in a radio interview that he is working to identify and fundraise for a third-party candidate.

National Review writer David French attacked Priebus for calling for unity and branded Trump "the destroyer of conservatism.”

Senate Majority Leader Mitch McConnell gave a tepid nod to Trump, again putting the onus of uniting the party on the billionaire.

“I have committed to supporting the nominee chosen by Republican voters, and Donald Trump, the presumptive nominee, is now on the verge of clinching that nomination,” McConnell said in a statement Wednesday. “As the presumptive nominee, he now has the opportunity and the obligation to unite our party around our goals.”

Meanwhile, there are signs the Democratic campaign machine is warming up for a huge general election battle. On Wednesday, the party’s official Twitter account declared: “It's time to get to work to make sure we never hear the words ‘President Trump.’”

Fox News’ Mike Emanuel and Serafin Gomez contributed to this report.