House Speaker Paul Ryan (Wis.) warned Thursday that his endorsement of Donald Trump was "not a blank check" and sharply criticized the Republican nominee’s campaign two days after Trump refused to endorse the Wisconsin congressman’s reelection bid.

Ryan affirmed that he supports Trump but would still consider withdrawing his endorsement in an interview with Green Bay’s WTAQ radio.

"I’m not going to get into the speculation or hypotheticals. None of these things are ever blank checks," Ryan said. "That goes with any situation in any kind of race. But right now, he won the thing fair and square."

Ryan added that he will do what is necessary to protect and preserve the Republican Party.

"If I hear things that I think are wrong, I’m not going to sit by and say nothing, because I think I have a duty as a Republican leader to defend Republican principles and our party’s brand if I think they’re being distorted," he said.

The House Speaker expressed concern that Trump is making too many unforced errors rather than focusing his attacks on Hillary Clinton’s weaknesses.

"He’s had a pretty strange run since the convention," Ryan said. "You would think that we want to be focusing on Hillary Clinton, on all of her deficiencies. She is such a weak candidate that one would think that we would be on offense against Hillary Clinton, and it is distressing that that’s not what we’re talking about these days."

Trump has come under fire over the past few days from key Republicans for attacking the family of a fallen Muslim American soldier, Capt. Humayun Khan, refusing to back reelection bids of party leaders like Ryan and Sen. John McCain (Ariz.), and booting a mother with a crying baby out of his rally.

Ryan reiterated Thursday his criticism of Trump’s attacks on Khan, calling them "beyond the pale." While Ryan said the controversy would not break his endorsement of Trump, he vowed to continue speaking out against the nominee’s controversial statements.

"I don’t like doing this. I don’t want to do this, but I will do this because I feel like I have to in order to defend Republicans and our principles, so people don’t make the mistake of thinking we think like that," Ryan said.

The House speaker dismissed Trump’s comments earlier this week that he was not "yet" endorsing Ryan in his upcoming primary. He said he was focused on clinching endorsements from employers in Wisconsin.