US President Donald Trump has trolled his political enemies in the first of three rallies over as many days as Democrats vied in neighbouring Nevada to be the one to challenge him in the November election.

Just minutes before Trump regaled a friendly crowd in Phoenix, Democrats watched billionaire and former New York Mayor Mike Bloomberg make his debut on the debate stage ahead of Nevada’s party caucuses on Saturday.

Trump took aim at the new target.

“I hear he’s getting pounded tonight – you know he’s in a debate,” Trump said about the man he has dubbed “Mini Mike” because of his short stature.

“I hear that pounding. He spent $500 million so far and I think he has 15 points. Crazy Bernie was at 30.”

He has gleefully derided Massachusetts Senator Elizabeth Warren as “Pocahontas” for her past claims of Native American heritage and claimed her presidential campaign had stalled.

“Fortunately she self-destructed anyway,” Trump said.

“We don’t care who the hell it is,” he said. “We’re going to win.”

Arizona is a 2020 battleground state, but Democrats think with a little luck, the state could be in play.

The president’s four-day, four-state trip features big-dollar fundraisers, back-to-back-to-back campaign rallies and a sprinkling of official presidential events.

It’s an unusually long domestic trip for Trump, who prefers to sleep in his own bed.

Next best: He’s sleeping each night at his own hotel just off the Las Vegas Strip and making day trips to California, Arizona and Colorado.