Democratic presidential candidates, led by Vermont Sen. Bernie Sanders, are starting the new year by stepping up their criticisms of former Vice President Joe Biden’s ability to defeat Republican President Donald Trump, directly confronting the national front-runner’s greatest strength.

Biden’s continued healthy lead in national polling, and his still-strong-but-not-dominant position in the early voting states of Iowa and New Hampshire is directly linked to the perception, backed up by polling data showing Biden with a healthy lead over Trump, that he is best positioned and best equipped to defeat the incumbent.

Sanders, Buttigieg and close allies of Massachusetts Sen. Elizabeth Warren have all launched attacks on Biden since the new year that are focused on convincing Democratic primary voters both that they shouldn’t vote for Biden and that Midwestern swing voters won’t want to. The strategy is designed to give voters whose first concern is electability ― a majority of the Democratic primary electorate, according to public polling ― to vote with their liberal-leaning hearts instead of their Trump-focused heads.

“Everyone thinks Biden is electable because everyone keeps saying how electable he is. I think it’s important to have the conversation,” said Rebecca Katz, a progressive Democratic strategist who has been critical of the former vice president. “There’s been no vocal progressive bullhorn presenting a counter-message.”

There are major questions about the effectiveness of these attacks, most of which the Biden campaign dismissed as old news. And so far, none of the campaigns have been willing to attack Biden with paid advertising ― which would dramatically increase the number of voters exposed to criticisms.

Mike Blake / Reuters Former Vice President Joe Biden and Vermont Sen. Bernie Sanders speak toward the end of the sixth 2020 U.S. Democratic debate, Dec. 19, 2019.

During an appearance on CNN Monday night, Sanders ticked off a number of potential attacks on Biden, noting his support for the North American Free Trade Agreement, the war in Iraq and his past support for cutting Social Security.

“You think that’s going to play well in Michigan or Wisconsin or Pennsylvania?” Sanders asked host Anderson Cooper, ticking off three midwestern states won by Trump that are largely seen as crucial to the eventual Democratic nominee’s path to 270 electoral votes.

Warren and Buttigieg have not been as direct. The Progressive Change Campaign Committee ― a group that has long functioned as Warren’s political enforcers ― questioned Biden’s electability after the Massachusetts senator unveiled a plan to roll back many of the provisions of a 2005 law making it harder to qualify for bankruptcy, arguing it would allow Trump to seize the “outsider mantle.”

“When thinking about electability, it would be complete malpractice to nominate someone who conspired in backrooms for years with credit card lobbyists and voted for every corporate bankruptcy bill, Wall Street deregulation, and trade deal that voters hate,” said Adam Green, the group’s co-founder.

Buttigieg, meanwhile, has criticized Biden over his support for the Iraq War and even questioned the former Vice President’s handling of his son Hunter’s work for Ukranian energy company Burisma ― an attack most Democrats have avoided.

Even entrepreneur Andrew Yang has gotten in on the action, hitting the self-styled “Middle-Class Joe,” for being out of touch with blue-collar workers. Yang, whose candidacy is premised on aiding workers made redundant by technology, seized on Biden’s suggestion that coal miners could “learn to program.”

“Someone who suggests that coal miners become coders is generally neither of those things,” Yang said.

The Biden campaign, meanwhile, is confident they can rebut these attacks. A long list of Democratic candidates ― California Sen. Kamala Harris, California Rep. Eric Swalwell and New York Sen. Kirsten Gillibrand, to name a few ― have dropped out of the race not long after lobbing an attack at Biden. They’re prepared to respond with examples of Sanders and Warren praising the former vice president: On Wednesday morning, a Biden endorser tweeted out a clip of Sanders praising Biden for devoting “his entire life to public service and to the well-being of working families and the middle class.”

If you want to know why I’m supporting @JoeBiden , Sen. Sanders actually articulated it very well.



He has dedicated his life to fighting for the middle class and Obama/Biden administration did make significant progress for our country.



That’s what we need now more than ever. pic.twitter.com/Mbhr6CU9nr — Rep. Malcolm Kenyatta 🥁 (@malcolmkenyatta) January 8, 2020

And they’re doubling down on their electability argument. On Sunday, after three members of Congress who won Republican-held House seats during the 2018 Democratic wave endorsed Biden, the campaign released a memo arguing Biden was in far better position than other Democratic candidates to drive down-ballot success and help Democrats win control of the Senate.

“This election is defined by the urgent need to defeat Donald Trump and the toxic values he represents. Poll after poll has shown that Vice President Biden would beat Donald Trump like a drum ― and Donald Trump proved he knows that when he became the only president in American history to attempt to extort a foreign country into lying about the rival candidate he fears the most,” Biden spokesman Andrew Bates said.

And Sanders’ attacks come as the Democratic establishment is warning about the Vermont socialist’s ability to beat Trump. Biden allies, in particular, suggested they would welcome a debate with their major challenger on foreign policy issues.

“Bernie’s the only presidential candidate who talked up the Soviet Union and said breadlines were indicative of a healthy economy,” one Biden ally said. “If he really wants to go there, God bless him.”

Electability has defined the 2020 presidential race, and several candidates have attempted to redefine the conversation around it, arguing Biden won’t be able to generate turnout among Black voters or young people. Those attempts have largely failed, and voters in Iowa and New Hampshire have remained focused on Midwestern swing voters. Sanders’ attacks, in particular, are designed to question Biden’s appeal there.

Veterans of Democratic politics are divided on whether the electability-focused attacks on Biden will have an impact, particularly given voters’ strong associations of the former vice president with former President Barack Obama, a sacrosanct figure in Democratic circles.

When it comes to questions of electability, perhaps no issue looms larger than Biden’s long history of support for corporate-friendly international trade agreements, including NAFTA, permanent normalized trade relations with China and the now-defunct Trans-Pacific Partnership. Many Democratic officials from the industrial Midwest and allied labor unions credited Trump’s fulminations about trade policy for helping him pick off just enough historically Democratic voters to put him over the top in Michigan, Wisconsin and Pennsylvania.

But Tim Waters, the former political director of the Pittsburgh, Pennsylvania-based United Steel Workers union, which was frequently at odds with Obama and Biden on trade policy, argued that Sanders’ and Warren’s attacks were liable to backfire.

“It seems a little desperate to me,” said Waters, who has not formally endorsed in the Democratic presidential primary. “Joe Biden was in the United States Senate for a long time … He has many decades of standing up for working families and trying to do the right thing.”

By contrast, Larry Cohen, a Sanders supporter and former head of the Communication Workers of America, which has also fought trade agreements that could jeopardize domestic jobs, insists that it is a relevant point of contrast. Cohen recently returned from a tour with the pro-Sanders groups Our Revolution and Labor for Bernie promoting Sanders in Eastern Iowa’s manufacturing-heavy cities and towns. The Eastern part of the state is also home to many of the counties that flipped from Obama to Trump.

“These cities from Dubuque to Burlington are devastated and people are wondering: Is there somebody who still believes in manufacturing jobs and standing up for the rights workers doing them?” Cohen said. “For a Democrat to have credibility they have to have the record. Vice President Biden ― lots of great things about his career ― his record on trade and foreign policy is not among them.”

Still, Biden’s greatest vulnerability may be on Social Security, which Sanders only mentioned in passing in his CNN interview on Monday night.

That may be a foretaste of future attacks. Following the interview, spokespeople for the Sanders campaign tweeted out video of Biden expounding on the need to reduce Social Security, Medicare and Medicaid benefits in a mid-1990s speech on the Senate floor.

More recently, Biden served as the point person for the Obama White House during budget negotiations with Republicans in which Obama offered to cut Social Security in exchange for tax hikes elsewhere. And in an April 2018 speech at the Brookings Institution, Biden declared that Social Security and Medicare benefits would need “adjustments,” including, he implied, lower benefits for affluent retirees.

Joe Radinovich, a former Minnesota House candidate and congressional staffer who hails from Minnesota politically purple-hued Iron Range, believes that a Social Security-driven critique could resonate in his home region.

In a notable departure from Republican orthodoxy, Trump promised to protect Social Security as a candidate in 2016. And though he has since sought to restrict access to Social Security disability benefits, he might well try to blur his differences with Biden on the issue, predicted Radinovich, who endorsed Sanders in 2016, but is unaligned this time around.

“His long history in Washington leaves him open to criticism that he’s been on both sides of every issue or been part of what Trump calls the swamp, and you can tie that to the false allegations of what Trump alleges about Biden and Ukraine,” Radinovich said.