Sen. Bernie Sanders, an independent from Vermont and 2020 presidential candidate, speaks during the Democratic National Committee (DNC) Summer Meeting in San Francisco on Aug. 23.

Bernie Sanders could use a breakout moment in his quest for the Democratic presidential nomination — one that could come at the expense of his progressive rival Elizabeth Warren.

The Vermont senator campaigned in early primary states this summer and added to his pile of far-reaching policy prescriptions, including a $16.3 trillion climate-change plan and an overhaul of the criminal justice system. But opinion surveys have him consistently and unrelentingly tied for second place with Warren — and not close to touching front-runner Joe Biden.

Sanders will be under increasing pressure at Thursday’s Democratic presidential debate in Houston to end a nonaggression pact with his Senate colleague Warren and emerge as the definitive second.

Democratic strategists say he is clearly challenged as he seeks to grow his support in a 2020 field that, unlike 2016, offers several progressive alternatives.

“His problem is very real,” said Robert Shrum, a former top adviser to Democratic nominees Al Gore and John Kerry, and now director of the Center for the Political Future at the University of Southern California. “In our data, Sanders and Warren are tied. And he’s not the fresh voice that he was in 2016. Sometimes if you close your eyes you wouldn’t know if it was 2019 or 2015.”

And Sanders can’t take for granted his backing among his base of people 18-to-29-year-olds, a demographic he carried by large margins in 2016. His support among that group seems to be hovering in the low to mid-20%, and he had trouble filling a venue at Iowa State University in Ames last weekend.

Sehba Faheem, president of ISU’s College Democrats, said Warren combines mainstream appeal with far-reaching ideas. “I think Warren offers a middle ground between what Hillary had before in terms of the experience and knowledge and the same large goals that Bernie has,” he said, referring to 2016 Democratic nominee Hillary Clinton. “When you stack them up, climate change, tuition, health care they stand shoulder to shoulder.”

Joe Trippi, a strategist who ran Howard Dean’s campaign for the Democratic presidential nomination in 2004, said Sanders could still emerge as the nominee to challenge President Donald Trump, but it’s a steep climb.

“There isn’t a clear path for him. That doesn’t mean there isn’t a path, but so far I haven’t seen it,” said Trippi, who isn’t aligned with any of the candidates. The field is so large that “they’re all in each other’s way.”

An ABC News/Washington Post poll released Sunday shows Biden, Sanders and Warren leading the field of candidates. Biden had support from 27% in the Sept. 2-5 survey of 437 Democrats and Democratic-leaning independents. Sanders had 19% and Warren had 17%, after she shot up 6 percentage points from an early July survey.

Jeff Weaver, a senior policy adviser to Sanders, said the race will evolve leading up to the first nominating contest, in Iowa on Feb. 3.

“This is a multi-candidate field, so this is not a race where you need to get 50% of the vote to win,” Weaver said. “It’s probably about 30% to 35%. I think as voters tune in, as they hear about a program that is going to uplift working people in this country, voters are going to remember that Bernie Sanders been the person talking about these issues for decades and that he is the best standard-bearer to take on Trump and take this country forward.”

Sanders had bright spots in some early primary state polling released over the weekend. He was in a virtual tie with Biden in Nevada and New Hampshire in an Aug. 28-Sept. 4 CBS News/YouGov survey of battleground states. In Iowa, he was 3 percentage points behind the former vice president and 9 points ahead of Warren. The poll also found, though, that he lagged Biden by 25 percentage points in South Carolina, where a majority of the Democratic electorate is African-American.

Sanders remains the money leader in the race. His ability to lure small-dollar donations helped him raise $18 million in the second quarter.

But his sweeping ideas on issues such as climate change, Medicare for All and using tax revenue to wipe out student debt continue to raise electability questions. Trippi says comprehensive policy proposals have propelled both Sanders and Warren, but the potential economic impact will get more scrutiny later as the field narrows.

Some leading figures in the Democratic Party insist Sanders has little to worry about. His consistent polling is a bright spot in a field where some of his rivals are ebbing and flowing — and some are flaming out, said Charles Chamberlain, chairman of Democracy for America, a grassroots progressive group with more than 1 million members.

Sanders is playing an aggressive ground game in states such as Wisconsin, Michigan, Pennsylvania and North Carolina, he said.

“One of his strengths is that they’re building a very strong campaign,” added Chamberlain, whose group hasn’t yet endorsed anyone. “He may not have risen in the polls, but they’re not sinking. They’re doing a fantastic job of steady as she goes, which is critical in a race with so many candidates as this.”

Biden continues to top Sanders with older voters, who historically have tended to turn out in higher numbers. And polls show Sanders’ support among black voters in the South, his undoing in his 2016 bid, continues to be a strong impediment in South Carolina, which holds a key early primary.

“We are coming a long way in building the kind of relationships we need in South Carolina and by extension the South,” said Nina Turner, a national co-chairwoman of Sanders’ 2020 campaign.

Sanders has campaigned 13 days in South Carolina so far, bested only by Kamala Harris, with 15 days. His visits have featured policy roll-outs geared toward black voters. That includes a plan to revamp K-12 education he laid out in May at an African American church in Orangeburg, S.C. and a criminal justice overhaul he proposed in August in Columbia.

Kwadjo Campbell, Sanders’ South Carolina state director, said there will be time to draw sharper distinctions with Warren, who unlike Sanders stresses her “capitalist” leanings and proposes ending just a portion of student debt.

“There are contrasts we can draw without getting into attack mode,” Campbell said. “It’s not who we are and it’s not how we’re going to run.”