WASHINGTON — President Trump said Friday that he plans to spends almost all of his time this fall campaigning for the most vulnerable Republican congressional candidates in the midterm elections, a strategy that would have him in many districts where endangered lawmakers in his own party regard him as a liability.

In a friendly radio interview with Sean Hannity, the conservative Fox News host with whom he shares a close relationship, Mr. Trump said he was confident the strength of the economy and the demise of the Islamic State would boost the fortunes of Republicans in this fall’s contests, and that he would personally work to pull lawmakers facing tough re-election challenges to victory.

“I am going to work very hard,” Mr. Trump said during the interview. “I’ll go six or seven days a week when we’re 60 days out, and I will be campaigning for all of these great people that do have a difficult race, and we think we’re going to bring them over the line.”

Mr. Trump said he had instructed John F. Kelly, his chief of staff, and others on his team to compile a list of about two dozen of the most hotly contested races in the country so he could use the bully pulpit to promote the Republicans running in them.