By JUANA SUMMERS, Associated Press

WASHINGTON (AP) — Bernie Sanders is quick to slam Joe Biden's past support of free trade deals and the Iraq War. He is warning him against a "middle ground" approach to addressing climate change. His campaign sends fundraising appeals with a simple, foreboding subject line: "Joe Biden ."

In his nearly three weeks as a presidential candidate , Biden has become Sanders' favorite foil.

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 skepticism in the party's progressive base.

The strategy is reminiscent of Sanders' approach to the 2016 Democratic primary, when he relentlessly slammed Hillary Clinton as an establishment pawn. And it's a reminder that, even when Sanders lags in the polls, he is often most comfortable when he's taking on top Democrats, hoping that such attacks will energize his most loyal supporters.

That was easy to do in 2016 when he was the sole outsider candidate taking on one of the most recognizable names in Democratic politics. It could be tougher now that he's a leading contender for the nomination who has spent the past several years building an organization to support his candidacy.

"Bernie is trying to rekindle the magic of 2016, where he was the outsider running against a longtime 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 Sanders' 2016 campaign, said the senator ran then by pitting himself against Wall Street, pharmaceutical drug companies and the billionaire Koch brothers, who funded conservative causes and campaigns.

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

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

Campaigning in New Hampshire this week, 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 Sanders and Democratic Rep. 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'll deliver a major speech on climate issues later this month, and he called for an "environmental revolution." But he also doubled down on 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, Biden is keeping his singular focus on President Donald Trump, a posture that also gives him the air of Democratic front-runner .

"You will never hear me speak ill of another Democratic candidate for president," Biden said Tuesday.

How long he can do that, though, is uncertain. With the first debates set for June, the race will soon move into another phase in which nearly two dozen candidates seeking the party's nomination will try to create breakout moments.

Zac Petkanas, a Democratic strategist who served as an adviser to Clinton's 2016 campaign, said Biden has the luxury of ignoring Sanders' attacks as the race right now appears to be "Joe Biden versus about 20 other candidates."

"It makes sense that he's trying to do it as long as he can, but we're in May of 2019. It's going to get rough, which is a good thing because we want a nominee to emerge battle tested," Petkanas said.

In New Hampshire, which Sanders captured by 22 percentage points in 2016 and his campaign views as vital this year, voters said they were wary of the campaign devolving into political mudslinging.

Lori Backman, 55, bemoaned the ideological tug-of-war, worrying that it will ensure Trump's reelection. "We can't have the splintering," she said, arguing that any Democrat is better on policy than the current administration. "We need a strong message of unity up front. That's how you win."

While Sanders benefited from running behind Clinton in 2016, 73-year-old Marilyn Learner said she didn't think Sanders would have that same advantage this time if Biden filled the Clinton role.

"Bernie's ideas were novel," the retired teacher said. "And they're not novel now."

Mike Ward, a 62-year-old retired postal worker, said Democrats should lay off one another for the time being but that he understands Sanders' approach.

"He's starting to slip in the polls," Ward said. "And it's due to Biden jumping in the race. That's obvious. So, he's just kind of scrambling to maintain his standing."

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Associated Press writers Elana Schor in Washington and Bill Barrow and Hunter Woodall in Nashua, New Hampshire, contributed to this report.