WASHINGTON — Four years ago, liberal activists and groups begged Senator Elizabeth Warren to challenge Hillary Clinton for the Democratic presidential nomination from the left, a request she ultimately declined.

But now that she’s running, many progressive groups and activists have instead lined up behind the liberal who ran against Clinton in her place, Senator Bernie Sanders. The flood of support for the 78-year-old democratic socialist in recent weeks boosts his chances in the still-fluid race and suggests he has the upper hand in their clash to become the progressive standard-bearer heading into the Feb. 3 Iowa caucuses.

Warren has the support of several prominent liberal figures and groups, including former presidential contender Julian Castro and the Working Families Party. But a series of strategic and policy moves, particularly on Medicare For All, along with her inability to land the most coveted liberal endorsement of the race — that of Representative Alexandria Ocasio-Cortez — appears to have cost Warren the chance to lock down the progressive wing of the party that Sanders has been courting for years.

“Her engagement with us just wasn’t as deep and hasn’t been as long,’’ said Bree Carlson, the deputy director at People’s Action, a grass-roots activist organization with a million members that endorsed Sanders in December. “We have a lot of history with Senator Sanders.’’

This month, the two chairs of the House’s progressive caucus, Representatives Pramila Jayapal and Mark Pocan, backed Sanders over Warren, praising his “authenticity’’ and joining Ocasio-Cortez and two other members of the so-called progressive “squad,’’ representatives Ilhan Omar and Rashida Tlaib, in his corner. Sanders also scooped up the endorsements from the youth-powered climate change group the Sunrise Movement, and the grass-roots activist organizations the Center for Popular Democracy Action and People’s Action, among several others.

The groups backing Sanders carry serious clout with the progressive voters both candidates have spent a year trying to consolidate. Losing these endorsements leaves Warren battling the perception that she has lost her mojo with the left at a time when she needs all the momentum she can get — as well as bereft of the practical organizational muscle they will provide to Sanders. A recent poll shows Sanders leading the field in Iowa, buoyed by support from voters who describe themselves as “very liberal.’’

His success with the left wing of the party this time around wasn’t always a foregone conclusion, given many activists’ deep respect for Warren and a feeling among some that Sanders’ time had passed.

But Sanders immediately set about building on the good will he earned after his 2016 run. The day before his campaign-launching speech in Brooklyn last year, he met with many of the groups’ leaders in person to ask for their support, making it clear that their backing was a top priority.

Leaders of the groups backing Sanders stressed that their members liked both candidates and were delighted to have two progressives to choose from, unlike in past elections. But Sanders was willing to go farther than Warren on some key issues, or at least adopt rhetoric that made members feel like he was more committed to their causes.

Warren’s more careful approach to policy in some areas, compared to Sanders’s, also appeared to put her at a disadvantage. Sanders and his team vowed to temporarily pause all deportations if he becomes president after receiving feedback from activists from the Center for Popular Democracy Action and other groups, a factor that helped him clinch the activist group’s backing. Warren said she was open to considering the move but has not officially embraced it.

“People came in expecting a grumpy white guy who knew everything and instead they got a candidate who truly listened to their concerns, who was deeply touched by the stories that they shared and was willing to go farther in his position,’’ said Jennifer Epps Addison, the group’s co-executive director.

Some pundits worry Warren is too liberal to win the general election, given her embrace of Medicare for All and public battles with Wall Street and billionaires. But to the smaller group of liberal activists who have spent years trying to push the party to the left, she has been judged as not being liberal enough.

Zina Precht-Rodriguez, the national political communications manager for Sunrise, said the group was looking for how fiercely and passionately each candidate spoke about the climate crisis, which ended up favoring Sanders.

Among other things, Sunrise’s activists were impressed with Sanders’s suggestion that fossil fuel executives should be held criminally liable for damaging the environment, even though Warren has already produced a plan that accomplishes a similar goal.

“That’s really aggressive language that we didn’t necessarily see from Warren,’’ Precht-Rodriguez said.

At certain points last year, it seemed like Warren could usurp Sanders on the left. Warren beat him to the punch on several progressive policy proposals, including her wealth tax and her plan to forgive student loans for most debt-holders. She also captured progressives’ attention last fall when she endorsed two liberal challengers taking on sitting Democrats in the House, a big statement in Washington, where backing challengers can alienate party leaders. (Sanders backed one, but not both, of those challengers.)

“That was a big signal to a lot of people in the progressive movement’’ said Waleed Shahid, a spokesman for Justice Democrats, a progressive grass-roots group that backed Ocasio-Cortez’s congressional run and is unaligned in the presidential primary.

In September, Warren landed the endorsement of the Working Families Party, the progressive labor-aligned group that was with Sanders in 2016, and was leading early-state polls while closing in on former vice president Joe Biden in national ones.

It appeared possible that she, and not Sanders, would emerge as the main progressive challenger to Biden.

Then everything changed.

Sanders’s heart attack in early October was closely followed by Ocasio-Cortez’s endorsement, vaulting him back into the limelight. Several progressive leaders, including Epps Addison and Precht-Rodriguez, mentioned the influence of Ocasio-Cortez, who has become a liberal social media juggernaut, on their memberships’ endorsement decisions.

“AOC is shaping the future of the progressive left in a lot of ways and definitely influences where these groups want to endorse,’’ said Shahid. “I think her endorsement meant a lot.’’

Meanwhile, Biden and Pete Buttigieg, the former mayor of South Bend, Ind., began hammering Warren over her Medicare for All plan. It forced her to lay out a detailed “transition’’ plan that aimed to open up Medicare for a couple of years as an option for Americans before moving to a fully public system. Her two-step plan began to spark doubts among some progressives, even though Sanders has never released a detailed plan for how he would implement Medicare for All.

“That was a big turning point,’’ Epps Addison said.

Warren has still attracted big endorsements from the left, and several major progressive groups have yet to endorse, including MoveOn, which launched a “Run Warren Run’’ campaign in 2014.

On Monday, a coalition of liberal groups that back Warren plan to roll out 3,000 new endorsements for Warren, including elected officials, economists, and activists as a show of strength for her on the left. But the list is of individuals, not major organizations.

The endorsements are not just symbolic.

The Center for Popular Democracy Action will knock on doors, conduct phone banks, and mobilize volunteers especially targeted at voters of color on Sanders’s behalf, through an independent expenditure arm. Sunrise has volunteers on the ground in early states who will now be advocating for Sanders and is asking young people to sign pledges promising to vote for Sanders.

As Sanders has piled up more liberal endorsements, Warren has leaned into a different strategy: pitching herself as a unity candidate who can rally together the left and center of the Democratic Party. From the beginning of the race, she distinguished herself from Sanders by pointing out that she believed in reforming capitalism with tougher rules, not abandoning it altogether.

Some Warren backers have suggested that liberal groups are afraid of raising the ire of Sanders’s fiercest online defenders by endorsing Warren, making Sanders a safer choice.

“It takes extra boldness to endorse Warren because of the vitriol from Bernie Twitter aimed at anyone who acknowledges how amazingly inspirational, progressive, and effective Warren is,’’ said Adam Green, the cofounder of the Progressive Change Campaign Committee.

Liz Goodwin can be reached at elizabeth.goodwin@globe.com. Follow her on Twitter @lizcgoodwin