Paul Ryan said there could be something Donald Trump would do or say to lose the House speaker's endorsement but that he didn't know what that would be. In an interview Friday with Wisconsin radio host Charlie Sykes, Ryan said no endorsement of a candidate comes with a "blank check" or an understanding that the two politicians agree on everything.

Sykes asked Ryan if the "check" had an "expiration date"—that is, could Trump say or do something to cause Ryan to reconsider his endorsement? "Is there anything that he could say that would cause you to basically say, I'm cancelling that check?" Sykes said.

"Of course," Ryan said. "But I'm not going to go down the road of relitigating his past comments."

The Wisconsin Republican said there was a hypothetical line that was too far for Trump to cross. "I don't know what that line is," he said.

Ryan defended his endorsement of Trump for president. He spoke about how Trump won the GOP primary "fair and square." "I feel as part of my responsibilities in this job…I have a duty and obligation to respect this process," he said.

He also rejected the idea that as a party leader, he needed to make a moral judgment on the party's nominee. "I don't have plans of having the moral authority of this or that," he said, adding that as speaker his obligation is to protect the GOP majority in the House.

Ryan is facing a primary opponent, businessman Paul Nehlen, who has praised Trump while denouncing Ryan. Trump himself has tweeted words of appreciation toward Nehlen and earlier this week declined to endorse Ryan in his primary, which is being held August 9. In his interview with Sykes, Ryan declined to call Nehlen by name, simply calling him "some people" or "this guy." Ryan also emphasized his own "born-and-raised" Wisconsin roots. Nehlen was born in Ohio.

Ryan told Sykes he has not spoken to Trump this week or at all since the Republican convention in Cleveland ended on July 21.