WASHINGTON, DC, USA (AP) — With firefighters chanting “Run Joe, run”, former Vice-President Joe Biden yesterday teased out themes of a possible presidential candidacy, criticising President Donald Trump’s leadership while questioning Republicans’ commitment to middle class America.

Biden, a Democrat, has not announced a presidential bid. But he asked members of the International Association Fire Fighters (IAFF) gathered in Washington to save their energy, because “I may need it in a few weeks”.

Then he cautioned them to “Be careful what you wish for.”

During a speech that lasted about 30 minutes, he presented a vision of America that he repeatedly contrasted with the politics — and president — of the moment.

While the “ugliness of extremism is on the rise”, Biden called on Americans to put aside the “mean pettiness” that has overtaken public discourse. He said the country doesn’t always live up to the founders’ ideals but is at its best when people keep trying.

“We can’t be divided by race, religion, by tribe. We’re defined by those enduring principles in the constitution, even though we don’t necessarily all know them,” Biden said. “In America, everybody gets a shot.”

Repeatedly, though, he tore into Republicans — and Trump — as supporting policies that favour the wealthy to the detriment of the middle and working classes. And he criticised Trump, who he said didn’t understand the country’s founding ideals “at all”.

Frequently, he returned to themes that were geared toward working class voters, whom he has criticised Democrats for ignoring, which he argues handed Trump the presidency.

“I’m a union guy. Labour makes a gigantic difference,” he said, questioning those who opposed collective bargaining rights by asking: “Who the hell are these guys?”

He singled out the Republican-championed tax cut signed into law by Trump, arguing that Republicans want to pay for it with devastating cuts to the social safety net.

“I’ll never understand people who fight you on the basics — like safety, fair play, health care,” Biden said.

Biden also questioned Trump’s “America First” foreign policy.

“Who fills the vacuum if we continue to walk away and walk off the world stage?” he asked. “This is America, so it’s time to get up remember who the hell we are.”

The speech was geared toward a pro-labour audience and Harold A Schaitberger, general president of the IAFF, says that the union is “all in” if Biden gets in the race.

Biden has working class cred and hails from Scranton, Pennsylvania. But there’s an open question as to how much he could win back blue-collar voters who abandoned the Democratic party for Trump, particularly in the upper Midwest.

Schaitberger acknowledged that many of his union members voted for Trump in 2016. But he argued that Biden’s is better positioned to win in the Midwest than other Democrats in the primary.

“You win the presidency with an electoral (college) victory,” said Schaitberger. “It doesn’t matter if you get six million more votes in California, and two million more votes in New Jersey, and a million and a half more in Massachusetts.”