Who will be Trump's number 2? Decision expected on Monday

With the pomp and pageantry of his official nomination just days away, Donald Trump's search for a running mate on the Republican presidential ticket is in the home stretch, with a trio of frontrunners jockeying to be first across the finish line.

The former US president is seeking a number two who can walk the delicate line between helping mobilize Trump's right-wing base and staying in lane as a sidekick who won't hog the limelight or, worse, prove more popular than the boss.

Trump is expected to make his choice public during next week's Republican National Convention in Milwaukee, with an announcement anticipated as early as the opening day on Monday.

The billionaire appears to have zeroed in on two US senators, J.D. Vance of Ohio and Marco Rubio of Florida, as well as North Dakota Governor Doug Burgum, who initially ran against Trump for the nomination before falling in line.

The calculus might yet change, as Trump told Fox News on Monday that he was weighing a "great bench" of prospects and had not made a decision.

"It's going to be a great vice president, meaning a person that can do a fantastic job as president, because you always have to think of that first," he said.

"And then, second, somebody that helps you get elected. And there's nothing wrong with that."

Ever the consummate showman, Trump has been ramping up the intrigue, sending out a "long list" of contenders early on and offering conflicting accounts of how far along he was in the selection process.

There was speculation that he might use his first public appearance in 11 days to announce Rubio as he addressed supporters at his Doral golf club, on the outskirts of Miami.

He namechecked the senator, who was in the front row, several times but was silent on his thinking for the veepstakes.

Trump could have dropped his big reveal at any point in the days before Milwaukee, except that the news would have interrupted arguably one of the worst weeks in the career of his 81-year-old rival Joe Biden.

The veteran Democrat is facing calls to drop out of the race after a widely-panned debate performance against Trump raised questions about the mental capacity of America's oldest-ever president.

"The big danger right now is because all the attention is focused on Biden, that if you announce anything, it will get lost," Republican strategist Rob Burgess, who worked on the Trump 2016 and 2020 campaigns, told AFP.

One of the biggest factors for Trump will be his running mate's fund-raising prowess, and the contenders have all been under pressure to showcase their connections with wealthy mega-donors.

Meanwhile, the campaign has ensured that they are booked regularly for cable news interviews -- a kind of casting call that allows Trump to assess how they look in front of the cameras, a priority for the former reality TV star.

"Although the vice president's role has grown, ultimately it depends upon the president's personality and his or her leadership style."