SALEM, N.H. — Bernie Sanders has long been a thorn in Joe Biden’s side, but strategists now say the Vermont U.S. senator’s ascendant campaign is proving a true threat to the former vice president’s Oval Office aspirations — a prospect that’s raising concerns among moderate-leaning voters in the Granite State.

“Joe Biden is afraid of Bernie Sanders and he should be afraid of Bernie Sanders,” said Republican strategist Mike Dennehy, an alum of the late U.S. Sen. John McCain’s presidential campaigns. “Bernie Sanders is surging, Bernie Sanders has momentum. Bernie Sanders is the most likable candidate among all the Democratic candidates for president. Joe Biden has to stop that if he has any hopes of winning the nomination.”

While Biden has maintained his lead in the national polls, the two are continuing to jockey for dominance in Iowa and New Hampshire just days before voting begins. Biden topped the most recent Franklin Pierce University-Boston Herald-NBC10Boston Granite State survey with 26% support to Sanders’ 22%, but the neighboring senator has the higher overall polling average in the state, according to Real Clear Politics.

“There’s a real issue here that Sanders can win the nomination, not just be a thorn in the side,” said veteran pollster John Zogby. “Right now it looks like Biden and Bernie, so much so that if Biden flags, then Bernie can either win the nomination or fight very hard with another moderate.”

Sanders has been ramping up his attacks on Biden for weeks, rekindling his criticism of the former senator’s 2002 vote in favor of the Iraq invasion and hitting Biden’s record on Social Security.

Biden’s campaign released a video Tuesday accusing Sanders of engaging in “dishonest attacks” in their Social Security feud. Friday in Claremont, N.H., he slammed Sanders’ climate plan, telling a voter, “Not a single solitary scientist thinks it can work.”

At a Saturday town hall in Salem, N.H., Biden implicitly continued to dig at Sanders for his 1990s votes against the Brady Bill requiring background checks for gun purchases.

“We all have plans, those of us who are running. But I put my record of getting consequential things done against any Democrat,” Biden said, citing “passing the Brady Bill” as one of his accomplishments.

Biden’s return fire comes as “Sanders is showing signs of durability, signs of strength here that Biden is not,” said University of New Hampshire political science professor Dante Scala. “Biden is chasing Sanders right now in New Hampshire. I don’t think it’s a tie.”

Voters at Biden’s Salem town hall expressed worry about Sanders’ resurgence.

“I’m shocked he’s gone this far,” Nashua, N.H., Democrat Mary Heil said of Sanders. “Biden has a wealth of experience and knowledge. That’s what we need.”

Hampstead, N.H., Democrat Lynne Snierson said she voted for Sanders in 2016 but is now leaning moderate. She’s concerned by moderates and conservatives who “tell me they would never vote for Bernie” but are part of the coalition “we need to win.”

Newbury, N.H., Democrat Jason Saghir cited Biden as one of his top two candidates and said, “With Bernie you get another divisive figure.”

Derek Summerville, a Dartmouth College alum, alluded to Sanders’ strong popularity among youths. But he wants the candidate “who’s going to get it done,” adding, “It might be time for the moderates to come home to Joe to make sure we seal the deal.”