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

Minutes before the president regaled a friendly crowd in Phoenix, Arizona, 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.

Mr Trump took aim at the new target, saying: “I hear he’s getting pounded tonight — you know he’s in a debate.

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

He derided 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,” he said.

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

Donald Trump arrives to speak at a rally in Phoenix
Donald Trump arrives to speak at a rally in Phoenix (Evan Vucci/AP)

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

The state is home to Republican senator Martha McSally, who stood by the president during the Senate’s impeachment trial.

In her rally remarks Ms McSally mentioned her Democratic opponent Mark Kelly, who has said he would back Bernie Sanders if he is the party’s nominee.

“Mark Kelly is flying on Bernie Sanders’ wing and I’m flying on your wing, President Trump,” she said.

Martha McSally speaks at the rally
Martha McSally speaks at the rally (Rick Scuteri/AP)

Mr Trump was on stage for more than an hour and 20 minutes.

Before he arrived at Arizona Veterans Memorial Coliseum, a crowd of protesters swelled into the hundreds. A large contingent of migrant rights advocates waved signs declaring “Dump Trump” and “Power to the people!”

After the president left, the protesters – in a “free speech zone” near the coliseum – were confronted by Trump supporters. Lots of yelling ensued as tensions flared, but there were no signs of violence.

On Thursday, Mr Trump will host a rally in Colorado Springs for another vulnerable Republican, senator Cory Gardner. On Friday, the president will appear at a rally in Las Vegas.

He did not wait until evening to start his counter-programming, weighing in on his Democratic rivals on Twitter. Earlier in the day, he took a swipe at Mr Bloomberg, who was criticised in 2016 for saying at Oxford University: “I could teach anybody – even people in this room, no offence intended – to be a farmer.”

The former New York mayor continued: “It’s a process. You dig a hole, you put a seed in, you put dirt on top, add water, up comes the corn.”

“Mini Mike hates the farmer,” Mr Trump said. “Never mind, I don’t think he’s going to be the candidate anyway, to be honest with you.”