A significant portion of Joe Biden’s supporters are open to voting for Bernie Sanders, and the Vermont senator’s new attacks show he knows those Democrats are up for grabs.

Such a strategy, if done successfully, could shoot Sanders to victory in Iowa.

Polling consistently shows Biden backers are most likely to pick Sanders as their second-choice candidate. A Morning Consult survey released Tuesday found 29% of Democratic primary voters who support Biden gravitate to Sanders as a second option.

“There are a lot of voters who are up for grabs. A lot of those voters who have expressed their preference for a candidate, it’s a fairly weak preference,” said Brad Bannon, a Democratic strategist. “The reality is that there are Biden voters who are still up for grabs, who can go to Bernie. There are Bernie voters out there, who could go for Biden. That’s not even counting the undecideds or the support that exists for the lesser candidates.”

Sanders started the new year with attacks on the former vice president, arguing that his “ baggage” would make him a weak candidate in a general election against President Trump.

Sanders’s team has often hit Biden for embracing the North American Free Trade Agreement and his 2002 vote in favor of the Iraq War. On Monday, Sanders targeted Biden’s past support for cutting social programs and spearheading a bill that made it harder to file for bankruptcy, issues that appeal to voters likely to cross from Biden to Sanders.

Many of those voters are older, male, and without a college degree. They also tend to trust both Sanders and Biden more than the rest of the field on issues such as the economy and foreign policy, despite the two candidates coming from vastly different ideological wings of the party.

“You know, Joe Biden has been on the floor of the Senate, talking about the need to cut Social Security or Medicare or Medicaid,” Sanders said Tuesday. “Joe Biden pushed a bankruptcy bill, which has caused enormous financial problems for working families. So, if we're going to beat Trump, we need turnout. And to get turnout, you need energy and excitement.”

A number of those who backed Sanders during the 2016 Iowa caucuses, which he lost to former Secretary of State Hillary Clinton by just a fraction of a percentage, have flocked to Biden’s camp. In that race, Sanders capitalized on Democrats’ resentment toward a candidate whose record appeared out of touch with voters. This time around, Sanders is hoping to do the same.

“At the end of the day, the people who are movable from Trump to the Democratic Party are for some reason also moved by Bernie and Biden. It could be the facade of Biden looking working class or Bernie actually talking about working-class issues,” said Nomiki Konst, a Sanders supporter and former member of the Democratic National Committee.

With a recent Iowa poll finding Biden and Sanders tied with former South Bend, Ind. Mayor Pete Buttigieg at 22%, a modest draw of soft Biden supporters could lead to significant success for Sanders in early nominating contests.

Of likely Democratic voters who backed Biden in a December Quinnipiac University national poll, 49% said that they might change their minds before voting in a caucus or primary.

“There are a lot more seniors going to caucus than voters under 35,” Bannon said when asked about Sanders’s new rhetoric on Social Security. “He has to go for those voters. You could shave 2% out of that pool and that would get you a victory in caucus.”

About 50% of Sanders supporters said that they might change their minds, while 76% of those who backed Massachusetts Sen. Elizabeth Warren and 83% who supported Buttigieg also said that their support was not set in stone.

Sanders was the top second-choice vote among Biden backers early in the primary cycle, but, for a short time in the fall, Warren beat out Sanders as the favorite second option among Biden supporters, coinciding with her bump in popular support for a few months.

“Now that Warren’s dropped back, what’s happening is that she’s less of a threat to Bernie now and the only thing standing between Bernie and a victory in Iowa is Joe Biden,” Bannon said.

Sanders’s attacks on Biden come as he is riding a comeback in support among Democratic voters, making his targeting of soft Biden supporters more feasible than ever.

On Tuesday, Sanders broke 20% support in RealClearPolitics’s average of national primary polls for the first time since late April 2019, at about the time Biden announced his candidacy.

He received in a massive $34.5 million in donations during the last three months of 2019, the best quarter for any Democratic candidate yet, $9.9 million more than Warren’s $24.6 million haul and $11.7 million more than the $22.7 million Biden raised over the same period.

“Folks who have been in the party for a long time, people in labor, they’ve been really familiar with Bernie. Biden represented that wing of the Obama coalition, and he had to do that work for Obama,” said Konst. “And I think what’s happening is that Bernie has been clear about where Biden really stands on working-class issues, and I think that’s why Bernie will be effective in winning those voters back from Biden.”