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-runners greatest strength.

Bidens 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 shouldnt vote for Biden and that Midwestern swing voters wont 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 its important to have the conversation, said Rebecca Katz, a progressive Democratic strategist who has been critical of the former vice president. Theres 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.