Party leaders are now openly questioning how Mr. Trump’s remaining Republicans rivals have approached his unusual candidacy and demanding that they have to let go of their restraint and take him on.

“It’s crazy that’s nobody else is trying to win except Trump,” said Mr. Stevens, who took direct aim at Senator Marco Rubio of Florida, long viewed by many within the party’s leadership as their best chance to overtake Mr. Trump.

“Rubio is not going after the person who is winning,” he said. “I’ve never seen a campaign that seems as satisfied to not go after the leader.”

The party’s collective shrug over Mr. Trump since he entered the presidential race last summer, and its stubborn unwillingness to treat him a serious threat, is reflected by the paltry sum that both campaigns and outside groups have devoting to undermining him.

In a presidential campaign during which “super PACs” spent $215 million, just $9.2 million, or around 4 percent, was dedicated to attacking Mr. Trump, even as he dominated the polls for months.