Bozeman, Montana - Donald Trump held his first campaign rally of the week as he continued to struggle to land a blow on his rival Kamala Harris during an appearance in Montana.

"I'd rather run against her. I think she's easier to beat, I really do," he assured the crowd in a typically rambling speech that lasted more than 90 minutes after a late start.

"I handed her the strongest border in US history, she turned it into the worst border invasion in the history of the world," he told a relatively sparse crowd.

Trump justified his stop in Montana, which he is heavily expected to win, as an opportunity to wrest back control of the Senate from Democrats.

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Endorsing Republican candidate Tim Sheehy, Trump said winning this seat means "we're going to win the Senate."

The ex-president, who survived an assassination attempt at a rally last month, has bristled at questions over his light campaign schedule.

In a hastily convened press conference Thursday, he said he had been absent from battleground states because he was "leading by a lot and because I'm letting their convention go through," a reference to the Democratic National Convention, which does not end until August 22.

Trump appeared on track to win back the White House before Biden dropped out of the race on July 21, but Harris has made big gains since replacing the president at the top of the ticket and announcing Tim Walz, Minnesota's governor, as her running mate.

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Her rise appears to have wrong-footed Trump, who dismissed her surging polling numbers as he held court in front of journalists at his Mar-a-Lago estate for a freewheeling hour-plus news conference.

He complained about coverage of Harris's large crowds and assailed her for avoiding interviews.

And in a bizarre moment that made headlines, he related an anecdote about a helicopter near-miss that never happened, in which he appeared to confuse former San Francisco mayor Willie Brown with Jerry Brown, California's governor for most of the 2010s.