Bernie Sanders has responded to Joe Biden’s entry into the race for the Democratic Party’s presidential primary campaign by targeting the former vice president’s support of free trade deals and the Iraq War.

He is warning him against a “middle ground” approach to addressing climate change while Mr Sanders’s campaign sends fundraising appeals with a simple, foreboding subject line: “Joe Biden.”

In his nearly three weeks as a presidential candidate, Mr Biden has become the favourite target of Mr Sanders who ran Hillary Clinton close for the party’s nomination in 2016.

No one seeking the 2020 Democratic presidential nomination has been as aggressive as the Vermont senator in highlighting episodes from the former vice president’s past to sow scepticism in the party’s progressive base.

Election 2020 Joe Biden
Joe Biden has replaced Bernie Sanders as leading Democratic Party candidate for the party’s presidential nomination (Michael Dwyer/AP)

The strategy is reminiscent of Mr Sanders’ approach to the 2016 Democratic primary, when he relentlessly slammed Hillary Clinton as an establishment pawn.

Even when Mr Sanders lags in the polls, he is often most comfortable when he is taking on top Democrats, hoping that such attacks will energise his most loyal supporters.

That was easy to do in 2016 when he was the sole outsider candidate taking on one of the most recognisable names in Democratic politics.

It could be tougher now that he is a leading contender for the nomination who has spent the past several years building an organisation to support his candidacy.

White House
The White House, Washington DC (Andrew Parsons/PA)

“Bernie is trying to rekindle the magic of 2016, where he was the outsider running against a long-time member of the establishment,” said Dan Pfeiffer, a former senior adviser to President Barack Obama.

“The challenge is that this year there are no candidates with a claim to outsider status.”

A Sanders representative declined to comment.

Mark Longabaugh, an adviser to Mr Sanders’ 2016 campaign, said the senator ran then by pitting himself first and foremost against Wall Street, pharmaceutical drug companies and the billionaire Koch brothers, who fund conservative causes and campaigns.

Mr Sanders’ critiques of Mr Biden come as the former vice president is taking the lead in many polls, displacing Mr Sanders from the top.

For his part, Mr Biden only nods at the tensions without mentioning Mr Sanders by name.

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The winner of the Democratic primaries will duel with Donald Trump for the keys to the White House (Evan Vucci/AP)

Campaigning in New Hampshire this week, Mr Biden defended his record as progressive, particularly on environmental and health care policies.

He pushed back at a news report that he was considering a “middle ground” on climate policy that prompted stinging criticism from Mr Sanders and influential Democratic Representative Alexandria Ocasio-Cortez of New York.

“I was in this area long before most anybody else was, and I have a record,” he said, calling himself “a leader in climate change” and referring to a 1987 Senate floor speech during which he referred to a warming Earth as an “existential threat”.

He said he will deliver a major speech on climate issues later this month, and he called for an “environmental revolution”.

But he also emphasised his overall pragmatic political brand, arguing that “we do need to finish this green revolution in a way that is rational” and in a way the nation “can afford”.

For now at least, Mr Biden is keeping his singular focus on President Donald Trump, a posture that also gives him the air of Democratic frontrunner.

“You will never hear me speak ill of another Democratic candidate for president,” Mr Biden said.