Bernie Sanders builds momentum with progressive groups ahead of Iowa caucus Securing these endorsements is important to rallying support, experts say.

With less than three weeks until voting starts in Iowa and beyond, the campaigns of both Sen. Bernie Sanders and Sen. Elizabeth Warren are jockeying for support from the progressive wing of the Democratic Party.

The two candidates have been friends for nearly 20 years and have, until now, maintained a pact of non-aggression during the 2020 race. Earlier this week, Sanders vehemently denied reports, including a statement from Warren herself, that he said a woman couldn't win against Donald Trump.

On Monday, Democracy for America, a progressive organization, called on the two candidates to cool down their rhetoric.

“Bernie Sanders and Elizabeth Warren, you both are progressive champions and our movement needs to see you working together to defeat your corporate Democratic opponents – not attack each other,” the group tweeted. “Progressives will win in 2020, but only if we don’t let the corporate wing or Trump divide us.”

However, the escalating of tensions between the two progressive, veteran lawmakers comes as Sanders builds momentum with a slew of progressive endorsements ahead of the Iowa caucus.

The Sanders campaign snagged endorsements within the past week from immigrant rights group Make the Road Action, youth-led environmental group the Sunrise Movement and advocacy group for people of color Dream Defenders.

“We can and we must fight for a country where members of our communities are free to stay, free to move, and free to thrive," said Javier Valdés, Co-Executive Director of Make the Road Action. "And we are proud to fight alongside Bernie Sanders to realize that vision.”

Sunrise Movement co-founder and executive director Varshini Prakash campaigned for Sanders in Iowa.

“We endorse Bernie Sanders because he stands for a Green New Deal,” said Prakash at a rally in Iowa City.

Prakash cited Sanders' pledge to address climate change, racial inequity, immigration and health care during her speech.

“Bernie’s track record is consistent,” read a statement from the Dream Defenders announcing their endorsement of Sanders’ presidential bid. “Throughout his career, he has pushed visionary ideas long before they were politically popular.”

Those endorsements are only the most recent.

The Sanders campaign can boast endorsements from the Center for Popular Democracy Action, People’s Action, and several labor unions. Rep. Alexandria Ocasio-Cortez, Rep. Rashida Tlaib and Rep. Ilhan Omar, members of the so-called “squad” of progressive freshman congresswomen of color, have stumped for Sanders on their home turfs and in early voting states.

“We earned those endorsements because of the long history of the senator fighting on behalf of and alongside these communities for issues they care about,” said Sanders' National Political Director Analilia Mejia.

“We communicated extensively in regards to policy platforms and in regards to issue campaigns that they were working on, and we made them part of the campaign, even before they endorsed, by way of soliciting their input,” Mejia said.

Securing these endorsements is important to rallying potentially millions of people behind a candidate, according to ABC News contributor and Democracy for America CEO Yvette Simpson.

“I think the progressive grassroots endorsements mean so much more than just $1, or a name on a website, or whatever,” Simpson told ABC News. “Because it comes with all of that power, real people ready to activate and mobilize behind these candidates.”

Simpson’s progressive organization endorsed Sanders in 2016, but hasn’t yet endorsed a candidate for the 2020 race.

The Sanders campaign did lose out on an endorsement from the Working Families Party to Warren in the fall. The group endorsed Sanders in 2016.

“Senator Warren knows how to kick Wall Street kleptocrats where it hurts, and she’s got some truly visionary plans to make this country work for the many,” Maurice Mitchell, the Working Families Party’s national director, told the New York Times in September. “We need a mass movement to make her plans a reality, and we’re going to be a part of that work.”

The Sanders campaign attributes the endorsement to timing. A person familiar with the Sanders campaign strategy said that including the input of grassroots organizations and elected officials across the country meant a slower rollout of policy proposals, many of which came after the Working Families Party endorsed Warren.

“They didn't benefit from our immigration policy rollout, our housing policy rollout,” said Mejia, who served as the executive director for the New Jersey Working Families Alliance prior to joining the Sanders campaign. “They didn’t benefit from a whole host of platforms that once we rolled them out, these organizations were reminded that Bernie was their guy.”

Meena Bose, executive dean at Hofstra University's Peter S. Kalikow School of Government, Public Policy and International Affairs, said Sanders is building a “very strong constituency” within the Democratic Party.

“Due to the health issues last fall, there was a time even into late last fall where it seemed as though perhaps the progressive wing of the Democratic Party -- the progressive supporters -- might look to Warren, that there was an opening there,” Bose said. “But after a very strong performance in the December debate [and] impressive fundraising, we've seen the endorsements. It’s pretty clear that Sen. Sanders is in this race to stay and go to battle for the nomination.”

Conversely, Simpson urged that with the uncertainty of the race, Warren shouldn’t be counted out.

“One of the things that you'll notice is that while Bernie has certain parts of the party, Elizabeth Warren has some of those parts of the party, too,” she said.

Warren has been endorsed by former 2020 opponent and former Housing Secretary Julián Castro, progressive activist Ady Barkan, and the only member of the squad who didn't endorse Sanders, Rep. Ayanna Pressley, and the Progressive Change Campaign Committee.

According to Sanders officials, more endorsements are to come -- the product of months of courting grassroots organizations, elected officials and communities.

“What [endorsements] mean for our campaign is that we expand our capacity, we expand our ability to talk to voters, expand our reach within target communities, and we do so with organizations and leaders who have the respect and trust of potential voters for the senator,” said Mejia.