MONTICELLO, Iowa — On his first trip to Iowa in his third bid for the presidency, Joseph R. Biden Jr. on Tuesday accused President Trump of coddling white supremacists, called former President Barack Obama “an extraordinary man” and vowed fidelity to longstanding Democratic priorities like Medicare and increasing the minimum wage.
He ignored his rivals for the Democratic nomination completely, not even hinting at any policy differences.
Senator Bernie Sanders, Mr. Biden’s closest competition in the polls, is taking a very different approach. Mr. Sanders went on CNN on Monday night to attack Mr. Biden for his history of supporting free trade measures, a deeply divisive issue in the party.
“I helped lead the fight against Nafta, he voted for Nafta,” Mr. Sanders said, before ticking off other trade pacts that he opposed and Mr. Biden backed. For good measure, Mr. Sanders added, “I voted against the war in Iraq, he voted for it.”
It was Mr. Sanders’s third broadside against Mr. Biden since he entered the race last week — and it marked a preview of the coming clash between the two septuagenarians that is likely to shape the early contours of the primary.
The competing strategies — Mr. Sanders targeting Mr. Biden while Mr. Biden wraps himself in the Obama legacy and excoriates Mr. Trump — illustrate the starkly different wagers the two candidates are making in the outset of the 2020 contest.
Mr. Biden’s bet is that his party’s rank-and-file consider the Obama years as a success and are animated chiefly by ejecting Mr. Trump. With a substantial lead in early polls, Mr. Biden is trying to remain above the intraparty fray, promising a new era of good feeling after the Trump interregnum and invoking his White House experience at every turn.
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“We got plenty of time to respond, I’m not going to get in a debate with my colleagues here,” Mr. Biden said Tuesday at an ice cream shop in eastern Iowa, gripping a chocolate-and-vanilla-swirl cone. He declined to directly answer Mr. Sanders’s critique, but did call himself “a fair trader” and, notably, said he did not regret his vote for Nafta as a senator.
The Trump administration represents “an aberrant moment in time,” Mr. Biden said Tuesday. Once the fever breaks, to borrow a phrase Mr. Obama favored, a consensus-seeker like Mr. Biden can heal the country and work with Republicans he once served with in the Senate to make Washington functional again.
“We have to talk about unity, we have to bring people back together,” he said to applause at a rally in Cedar Rapids, his first stop in Iowa.
Mr. Sanders, the leader in early polls until Mr. Biden joined the race, feels less urgency to unify the country or even the Democratic Party and has taken a far more aggressive posture toward the front-runner than any other contender. In targeting the former vice president, he is hoping to elevate himself as the leading progressive alternative in a 20-person field filled with candidates hungry for attention.
Mr. Sanders is running on the assumption that Democratic voters not only want to defeat Mr. Trump but also seek to shed the incremental, within-the-system politics of Mr. Obama and Mr. Biden as well as Bill and Hillary Clinton.
The current president is only a symptom of a party and a system that must be thwarted and not accommodated, Mr. Sanders believes, and this moment of economic and racial injustice demands bold, left-wing solutions.
Tactically speaking, this moment also demands that Mr. Sanders consolidate his support on the party’s left flank, and that explains an offensive against Mr. Biden that retraces some of his most effective lines of attack against Mrs. Clinton in 2016.
Mr. Sanders criticized Mr. Biden last week for beginning his campaign with a pricey fund-raiser at the Philadelphia home of a Comcast executive. He pre-empted Mr. Biden’s first speech as a candidate Monday by boasting that he was the only candidate in the race to oppose Nafta, which enables free trade among the United States, Canada and Mexico, and then made his trade critique more explicit Monday night before adding that their differing votes on the Iraq war as something of a bonus.
A new national poll of Democratic primary voters helps illustrate why Mr. Sanders is so eager to pick a fight with Mr. Biden: The former vice president has gained ground since entering the race and enjoys the support of 39 percent of his party’s voters while Mr. Sanders is in second place with 15 percent.
There are other reasons that Mr. Sanders is trying to frame the race in one-on-one terms. The same poll showed that Mr. Biden, a mainstream progressive, also enjoys a wide lead right now with liberals, with 32 percent compared with Mr. Sanders’s 19 percent. While the two men are seen by political insiders as hailing from starkly different wings of the party, here in Iowa, there is considerable overlap in their early bases of support.
A Des Moines Register-CNN survey last month of likely Iowa Democratic caucusgoers indicated that Mr. Biden had an overall advantage over Mr. Sanders within the margin of error, but more revealing is that their supporters named the other man as their preferred second choice.
That’s partly the result of their being the two best-known candidates at this early stage of the race. But the 2016 presidential primary, in both parties, demonstrated that assumptions about voters falling along expected ideological lines can be mistaken.
Longtime Iowa Democrats, however, note that there is another precedent that Mr. Sanders must be mindful of: The candidate who goes on the attack in the state is often not the one who reaps the benefit when the candidate being attacked drops in the polls.
“You’ve got to be careful because we don’t like that,” said Dave Nagle, a former Iowa Democratic chairman. He recalled how Richard A. Gephardt’s attacks on Howard Dean, the front-runner in the 2004 race, only helped John Kerry complete his stunning comeback. “I don’t think it’s helpful for the senator from Vermont to go after a combatant from the same army at this point,” Mr. Nagle said, referring to Mr. Sanders.
As for Mr. Biden, the only Democrat he is eager to discuss at the moment is the one who made him vice president.
On Tuesday morning, his campaign issued a video featuring the speech Mr. Obama delivered when he bestowed the Presidential Medal of Freedom on Mr. Biden (Mr. Obama granted his permission for Mr. Biden to use the footage, according to a Biden campaign official).
Asked in an interview on the ABC program “Good Morning America’’ where he would differ with Mr. Obama, Mr. Biden responded by emphasizing their shared history. “I’m really proud to have served with Barack,” he said.
And he interrupted himself during his torpid remarks to a few hundred Iowans in Cedar Rapids to testify to Mr. Obama’s character in the face of adversity.
“Everything but locusts landed on his desk,” Mr. Biden said, drawing a loud ovation in the state that helped propel Mr. Obama to the Democratic nomination in 2008.
At the Cone Shoppe in Monticello, Mr. Biden said that he was proud of his record and that there would be “plenty of time on the stage” of the party’s debates to engage with his rivals.
But at the moment, he indicated that he was more interested in dairy than disputes over free trade.
“I’m a free ice cream eater,” he told a reporter, changing the subject and laying down a pair of $20s to make clear he didn’t mean it literally.