As Bernie Sanders targets Joe Biden's supporters in a series of new campaign speeches, the Vermont senator is hoping to get a boost in Iowa and New Hampshire from Elizabeth Warren's struggling campaign.

When she first announced her White House run, Sanders's campaign and his supporters were initially dismissive, particularly after her abysmal early fundraising. Conversely, Warren's campaign was perfectly happy letting Sanders take the brunt of criticism from centrists in the party over his Medicare for All plan, which she co-sponsored.

After Warren began rising in the polls, however, individuals in and close to the Sanders campaign began fretting that they had no strategy to counteract her message of policy-driven populism. At one point in the race, in October, some polls showed Warren enjoying a double-digit lead over Sanders. Members of liberal groups began questioning if Warren had secured the left-wing mantle from Sanders and whether it was time for him to drop out.

For a brief few weeks this fall, just after Sanders suffered his heart attack in October, the Massachusetts senator made an aggressive push to court unions and liberal groups that were otherwise considered safely in Sanders's camp. Those concerned about the left-wing vote being split increasingly began blaming Sanders's stubbornness and insistence on remaining in the race.

Yet, Sanders quickly recovered from his health problems and began holding rallies that far outnumbered Warren's at the peak of her support. Members of the Democratic Party such as Reps. Alexandria Ocasio-Cortez and Ilhan Omar announced endorsements of his candidacy, signaling that the liberal base of the party still supported him. At the same time, left-wing members of the party began questioning Warren's sincerity about her commitment to Medicare for All.

For now, Biden maintains a sizable lead over Sanders in national polls, even as Sanders surges in Iowa. Surveys released this month show Biden beating Sanders by anywhere from seven to 13 points. In those same polls, Warren has clung to the top of the pack with about 15 points but is still behind Sanders, who averages 20% support.

According to a RealClearPolitics average of recent polls, Sanders and Warren have more than 35% combined support, compared with Biden's 29.3%. Other polls released late last year also show that Warren voters rank Sanders as their second-choice pick. Those same polls also show that a high number of Biden's supporters are also open to voting for him, meaning Sanders could form a majority coalition of Democratic primary voters to secure the nomination.

For now, however, Sanders and Warren are victims of their success. Each campaign is planning on the demise of the other to defeat front-runner Biden. Both sides have also hesitated from criticizing one another out of fear of alienating their supporters.

"I've been in talks with the Working Families Party, which endorsed Warren, on agreeing that neither campaign will attack one another," said liberal activist Randy Bryce, who supports Sanders. "It's a cordial relationship. I don't look at her campaign as a threat, we want the best progressive to win."

Because Warren's campaign strategy requires a bulk of Sanders supporters to join its side, they have remained focused on attacking Pete Buttigieg on issues such as transparency and donors. But that strategy has seemingly backfired on Warren as the former South Bend, Indiana, mayor has only seen his numbers rise, while hers continue to fall.

Warren's task will also prove more difficult as her pitch to Buttigieg voters seems less ideological and more personal. Whereas Sanders has been attacking Biden for his voting history and positions on issues, Warren's attacks on Buttigieg's fundraising appears increasingly desperate as she moves to the back of the front-runner pack.

Meanwhile, Biden continues to benefit from a crowded field. As his rivals jockey for support, no candidate has yet to maintain a national lead over him.