Countdown to Their Capitol Hill Meeting: Inside the Donald Trump–Paul Ryan Dynamic

— -- For the first time since he announced his White House bid, presumptive GOP presidential nominee Donald Trump will meet with the leadership of the House of Representatives.

Enter Speaker Paul Ryan, who has not yet endorsed Trump. The pair met once, when Ryan was Mitt Romney’s running mate during the 2012 presidential election. Yet on the campaign trail, Trump up until recently, had occasionally taken hits at the newly-elected speaker.

During the chaos on Capitol Hill last fall, Trump was asked if he would support the Wisconsin representative for speaker. Trump, dodging the question, said "Paul Ryan's a nice man. I would like to see someone extremely tough in there, but that vote’s not up to me, it it's up to the people who are voting.”

Trump’s key issue on the campaign trial has been immigration. Last year Trump said Ryan was "weak on illegal immigration," and he has also criticized Ryan's stance on amnesty.

At a rally in South Carolina in February, Trump also blamed Ryan for Romney losing the 2012 presidential election.

"Every single other candidate is going to cut the hell out of your Social Security -- remember the wheelchair being pushed over the cliff when you had Ryan chosen as your vice president?" Trump said at the time. "That was the end of that campaign, by the way, when they chose Ryan. And I like him, he's a nice person, but that was the end of the campaign. I said, 'You've got to be kidding -- because he represented cutting entitlements...The only one that's not going to cut is me."

Trump and Ryan had a phone call in March to discuss the House Republican's planned election-year agenda. Trump said at the time during a press conference in Washington that Ryan “couldn’t have been nicer.”

Ryan told reporters Tuesday that he and Trump "just need to get to know each other" and work together to find common ground now that the real estate mogul is the GOP's presumptive presidential nominee.

The Wisconsin Republican has frequently rebuked Trump's campaign, including his proposed ban of Muslim immigration, and Trump's initial resistance in denouncing the Ku Klux Klan after he was endorsed by former grand wizard David Duke.

“If a person wants to be the nominee of the Republican Party, there can be no evasion and no games," Ryan told reporters at a weekly House GOP leadership news conference in March. "They must reject any group or cause that is built on bigotry. This party does not prey on people’s prejudices,"

He has repeatedly declined to detail what Trump needs to do to earn his endorsement.

Asked about some of his disagreements with Trump on policy, Ryan said the GOP is a big-tent party with room for disagreement, and focused on the big picture.

"The goal here is to unify the party around common principles,” he said. “We have an obligation to merge and unify around these common principles to offer this country a choice, a better way forward.”

ABC News' Ben Siegel and Mary Bruce contributed to this report.