Speaker Paul Ryan suggested Thursday that his endorsement of Donald Trump for president is not written in stone.

In a radio interview with WTAQ's "The Jerry Bader Show," the Wisconsin Republican was asked if there would ever be a bridge too far when it comes to the GOP presidential nominee.

"None of these things are ever blank checks," said Ryan, who then added that Trump won the Republican nomination "fair and square" from the voters and delegates.

At the beginning of the interview, Bader asked Ryan how he can continue to walk such a fine line from now until the election of still backing Trump, while constantly criticizing his remarks.

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"We are a party where the grassroots Republican primary voter selects our nominee. That's as it should be. That is what it is. I think something should be said about respecting those voters," Ryan said. "He won it fair and square."

Ryan brushed off Trump's comment from earlier this week in which Trump said he's "not quite there yet" as far as endorsing Ryan for reelection. Ryan said he's only interested in endorsements from employers in his congressional district. Paul Manafort, Trump's campaign chairman, told "CBS This Morning" on Thursday that Trump plans to support Ryan.

"He's going to support Paul Ryan, he does support Paul Ryan, he said he's going to work with Paul Ryan. There's no issue about that. He didn't take a position in the primary, he didn't take a position in many primaries. That's not the news. The news is the two of them working together to elect a Republican Congress and a Republican president, and I think you're going to see that," Manafort said.

Since Ryan endorsed Trump in June, he said Thursday that he has not shied away from calling Trump out if it's warranted and will continue to do so.

"I said at that time, 'If I see a situation where our conservative principles are being distorted, I'm going to stand up for those conservative principles,'" Ryan said. "I'm not going to sit by and say nothing. That's what I said I would do, and that's what I'm continuing to do."

Asked again if there ever would be a point at which Trump could go too far, Ryan said, "Of course there is. That's with any situation like this."

He also said that Trump has had "a pretty strange run since the convention" in Cleveland last month and that it's a shame that Republicans aren't focusing on Hillary Clinton.

"It is distressing that that's not what we're talking about these days," Ryan said.

Ryan praised Trump's vice presidential nominee Mike Pence for being part of the conservative movement while serving in the House and how he fought against "the earmark machine" with him and supported his early budget blueprints.

"We were part of the original insurgent movement in Congress," Ryan said.

Ryan also slammed his primary opponent Paul Nehlen, saying that "he's been making stuff up" and hoping it will stick with voters in Wisconsin.

"He's running this sort of make-it-up-as-you-go campaign," Ryan said.

Trump will hold a rally in Green Bay, Wisconsin on Friday afternoon and the state's three top Republicans -- Ryan, Gov. Scott Walker and Sen. Ron Johnson -- are not expected to attend.