Donald J. Trump on Tuesday refused to endorse two leading Republicans — the House speaker, Paul D. Ryan of Wisconsin, and Senator John McCain of Arizona — in their primary election campaigns.

Mr. Ryan and Mr. McCain have both endorsed Mr. Trump for president, but they have criticized his denouncement of Khizr and Ghazala Khan, the parents of Capt. Humayun Khan, who was killed in Iraq in 2004.

“I like Paul, but these are horrible times for our country,” Mr. Trump told The Washington Post. “We need very strong leadership. We need very, very strong leadership. And I’m just not quite there yet. I’m not quite there yet.”

Mr. Trump had signaled support for Mr. Ryan’s little-known primary opponent, Paul Nehlen, in a Twitter message on Monday night, thanking him for his “kind words.”