WASHINGTON — Joe Biden and Bernie Sanders, front-runners for the Democratic presidential nomination, are the odd couple of the 2020 campaign.

Sanders is a rumpled, gruff socialist who hasn’t registered as a Democrat. Biden is well tailored and genial, a capitalist and party regular.

They both want to beat President Trump but disagree on a central question: How much will that, alone, do to cure what ails America?

Biden calls Trump’s election in 2016 an “aberration,” a political mistake that can be corrected by ousting him in 2020. His nostalgia-infused campaign, which often harks back to the good old days of Barack Obama’s presidency, calls for bolstering the middle class, not blowing up the system.

“I believe history will look back on four years of this president and all he embraces as an aberrant moment in time,” Biden said in announcing his candidacy.

For Sanders, Trump’s election marks the culmination of growing problems in the U.S. that can’t be solved without upending a system that was rigged long before Trump. The senator from Vermont wants a political revolution, not a return to the Obama era.

“Make no mistake about it, this struggle is not just about defeating Donald Trump,” he said in his first rally as a candidate. “This struggle is about taking on the incredibly powerful institutions that control the economic and political life of this country.”

That disagreement sets the stage for conflict on major issues: Sanders has already criticized Biden on trade policy, the Iraq war and corporate donors. Biden has not attacked Sanders himself, but his allies dismiss Sanders’ proposals to replace the current healthcare system and to give voting rights to violent criminals in prison as political losers.

“Those are the extremes that ultimately hurt the party and will set up a candidate to lose,” Biden supporter Harold Schaitberger, president of the International Assn. of Fire Fighters, said in an interview.

The 21-candidate field remains extremely fluid with more than nine months to go before voting starts. For now, however, national and key state polls consistently have placed Biden and Sanders at the top. They were also two of the top three fundraisers for the first quarter of 2019.

Until those facts change, the two men are boulders blocking the progress of other candidates looking to break into the top tier.

The good news for the rest of the candidates is that so many Democratic voters remain undecided, and their views probably fall somewhere between Biden’s reform message and Sanders’ revolutionary cry.

“The loudest voices in the Democratic Party agree with Bernie’s assessment, but maybe the more numerous voices are more likely to accept Biden’s account,” said William Galston, a centrist Democrat who was an advisor to President Clinton. “For a lot of Democrats, if you could wave a wand and restore the Obama administration, they would be quite content.”

If the two leading candidates damage each other in early squabbling, that could open a path for other candidates to rise and win support from voters who dislike both front-runners, such as Aaron Hernandez, a 26-year-old who sported an Obama-Biden T-shirt at a recent Iowa event for Sen. Elizabeth Warren of Massachusetts.

Hernandez said his support for Obama does not transfer to his former vice president, but he is also wary of Sanders because of his age and shortage of policy specifics.

“He is really old,” Hernandez said of Sanders. “My problem with Biden is his politics very much came up in the neoliberalism of the 1990s.”

It is ironic for a 2020 candidate field much touted for its unprecedented array of gender, racial and generational diversity to start out topped by two septuagenarian white men.

Although Sanders and Biden are 77 and 76, respectively, they draw support from opposite sides of a yawning generation gap. Sanders is buoyed by support from younger voters, while Biden dominates among older ones.

Both are trying to make the case that they are the one best equipped to beat Trump by appealing to disaffected Democrats and independents who voted for Trump. The Scranton-born Biden says his Pennsylvania roots and union ties will help him win the key industrial states that were lost to Trump in 2016. Sanders is targeting those same voters with his populist message and by opposing free-trade agreements that he says have battered the industrial economy.

“Right now, it’s fair to say they are both equally well positioned to beat Donald Trump,” Sanders’ campaign manager, Faiz Shakir, says. “But you have to have authenticity and conviction on the issues, and Bernie Sanders has a lifetime of consistency on these matters.”

Sanders prides himself on having held the same views throughout his 38 years in public life. He proudly notes that ideas that were thought radical when he ran for president in 2016 — a $15-an-hour minimum wage, “Medicare for all” — are now widely held among Democrats.

Biden has had a mostly liberal record over his four decades in Washington, but it includes past positions out of step with the current party consensus, including his support for the war in Iraq and for anti-crime measures now criticized as contributing to over-incarceration of black Americans.

He has moved to the left of where Hillary Clinton stood on some issues: He embraced the $15-an-hour federal minimum wage, for example, and has said he does not want outside support from super PACs and will accept no donations from corporate political action committees or federal lobbyists.

One of the clearest differences between the two men’s worldviews comes in their approach to healthcare: Sanders is the premier advocate of Medicare for all and is unafraid of the idea’s most disruptive feature — the end of private health insurance, an industry he believes puts profits ahead of providing affordable care.

Biden, by contrast, would allow people to buy into Medicare if they want it but would keep private insurance for those who prefer. That’s a more incremental approach, but, in a reflection of how the politics of healthcare has shifted, it’s a large increment: Biden’s plan would go considerably further than the “public option” that Obama could not get through a Democratic-majority Senate during the debate over the Affordable Care Act in 2010.

Sanders and his campaign have taken aim at Biden with barbs designed to portray him as too close to corporate interests. Shakir, in an email to Sanders supporters, poked Biden for holding his first fundraiser at the home of a Comcast executive in Philadelphia.

Trade policy has provided another point of difference. In both political parties, the issue has split elite economic interests that favor globalism away from labor unions and workers in industries that favor protectionism.

In a campaign video and an interview on CNN last week, Sanders pointed to the contrast between him and Biden on trade policy: Sanders was against and Biden was for the North American Free Trade Agreement, expanded trade relations with China and the Trans-Pacific Partnership — a 12-nation trade deal negotiated by Obama.

“I like Joe Biden. Joe is a friend of mine,” Sanders said. “But I think what we need to do with all of the candidates, have an issue-oriented campaign, not personal attacks, but talk about what we have done in our political lives, what we want to do as president, and how we’re going to transform our economy so that it works for all of us and not just the 1%.”

Biden has steered clear of commenting on Sanders or any other rival. Asked about Sanders’ criticism of his record on trade, Biden said while campaigning in Iowa: “We got plenty of time to respond. I’m not going to get in a debate with my colleagues here.”

When asked whether he was a “free trader,” Biden replied: “I’m a fair trader. That’s why I’ve been arguing for a long time that we should treat other countries the way in which they treat us, which is, particularly as it relates to China: If they want to trade here, they’re going to be under the same rules.”

While Biden so far has been sparing in policy specifics, he has been liberal in his references to Obama, tapping into primary voters’ fondness for the former president and to their weariness with the turmoil and policies of the Trump era.

Although Obama has not endorsed Biden or any other candidate, the former president’s voice provided most of the soundtrack of Biden’s second campaign ad. The voice-over came from Obama’s remarks describing Biden’s accomplishments as vice president when he awarded Biden the Medal of Freedom in January 2017.

“He’s nowhere close to finished,” Obama foretold.

At one of his campaign events in Iowa, Biden basked in the glow of his role in the Obama administration.

“Folks, I want to stop here and say something that I don’t think is said often enough,” Biden said. “President Obama has extraordinary character — no, really I mean it — extraordinary integrity and is a man of absolute decency. When he was president, our kids had someone to look up to. I was proud every day, every day, as I stood next to him as his vice president.”

Whether that pitch wins him votes or not, it might be political Valium for Democrats traumatized by the Trump era.

“The American people feel battered and bruised by endless controversy,” Galston said. “There may be a yen for some normality.”