HOLLIS, N.H. — Bernie Sanders vaulted to the top of the Democratic field by dominating the debate and controlling the agenda, party stalwarts believe, even as they harbor doubts about the Vermont senator’s viability.

Joe Biden, the only Democrat to consistently out-poll Sanders, launched his campaign Thursday. The former vice president will no doubt affect the trajectory of the race. But it is a 77-year-old socialist who has been the axis around which each of the other nearly two-dozen candidates revolve.

“Every candidate in the race, wittingly or unwittingly, has been contrasting themselves with Bernie, not with Biden. He has been the center focal point,” Joe Trippi, a veteran Democratic strategist, told the Washington Examiner.

Sanders has defined the first five months of the Democratic scrum. Nearly every candidate has faced comparisons to the senator: On issues, particularly healthcare, a high priority for the liberal base; on age and the generational choice before Democrats; on philosophy; and on who might be the most effective nominee against President Trump — a centrist or a liberal.

The front-runner’s unique influence on the campaign, not to mention the party, is unmistakable to grassroots Democrats.

Party activists in New Hampshire, a key early primary state, credited Sanders with almost single-handedly beginning and now reviving the 2016 leftist challenge to the Democratic status quo. In that last primary, the senator waged a surprisingly vigorous fight for the nomination against establishment favorite Hillary Clinton.

“The political stage changed in 2016 because of Bernie,” said Anne DiCicco, 60, a leader of the local Democratic affiliate in Hollis, a leafy community of upscale homes at the southern end of the Granite State. DiCicco, a restaurateur who is neutral in the primary, said Sanders has accumulated goodwill because many Democrats believe he transformed previously fringe ideas such as “free college” and “Medicare for all” into mainstream proposals, and not just on the Left.

“The other candidates have just seen how popular these things are and they’re like, we need to get on board because this is the direction the new party is going to take us,” DiCicco said.

Another Hollis Democrats member, a retiree named Jane who declined to let the Washington Examiner use her last name, was more blunt about Sanders’ effect on the primary. “So many of the other candidates have adopted his ideas,” she said. To many Democrats paying close attention, Sen. Elizabeth Warren, D-Mass., is the exception, with fresh policies unveiled seemingly everyday.

Most Republican operatives are giddy about the prospect of a Sanders nomination, viewing the senator, a member of Congress for nearly 30 years, as too liberal, both tonally and substantively, for average voters. Some Democratic insiders share that perspective. They are alarmed by Sanders' political strength, fretting the septuagenarian is unelectable.

But many experienced Democratic operatives, whether publicly supportive of Sanders or one of his opponents, caution against jumping to conclusions.

The senator benefits from high national name recognition and a loyal fundraising and activist base already in place when he kicked off his 2020 campaign, giving him a crucial advantage over competitors who are essentially assembling their operations and cultivating supporters from scratch. As more candidates enter the primary, and as existing contenders accelerate their campaigns, Sanders’ soft political underbelly could be exposed.

The senator has vulnerabilities that Clinton never challenged aggressively as she attempted to avoid elevating the then-underdog candidate. These include his age, his lack of foreign policy knowledge, his potential problems with nonwhite Democrats, or even his domestic policies. Sanders cannot count on the same kid-gloves treatment from a crowded field of ambitious Democrats. Debate season, beginning in late June, is the inflection point many in the party are anticipating.

“Regardless of what the polls show today, everybody’s numbers reflect name identification more than anything. We’ve seen this time and time again,” said James Demers, a longtime Democratic operative in Concord, N.H., who backed Obama early in the 2008 campaign and endorsed Sen. Cory Booker, D-N.J., in this race.

With the high-profile Biden in the race, Sanders cannot as easily be the defining candidate he has been. “That changed today,” Trippi said Thursday.

Biden has led Sanders in national polling averages by as many as 8 percentage points, and the two are jockeying for advantage.

Sanders’ handling of Biden could be an early test of his staying power in the top tier. Appreciated for eight years of service to Obama, Biden is just as well known and immediately competes with Sanders' ability to attract resources and media coverage.