This is not the first time Messina has questioned Sanders' viability as a general election candidate. His latest remarks come amid Sanders’ first-place showing in the marquee Iowa Poll and as the Vermont senator‘s messaging has increasingly focused on his electability.

Messina said he is not endorsing in the 2020 race. He recently attended a fundraiser for Biden in Irvine, Calif., he said, because his wife is a supporter of the former vice president.

“From a general election perspective, socialism is not going to be what Democrats are going to want to defend,” Messina added.“If you’re the Democratic nominee for the Montana Senate race, you don’t want to spend the election talking about socialism.”

Messina is the latest Democrat to raise concerns about Sanders at the top of the ticket. Endangered House Democrats are coalescing around Biden because of concerns that Sanders or Elizabeth Warren could threaten their reelection hopes, POLITICO reported Saturday.

Sanders’ campaign pushed back by pointing to news reports in 2008 saying that Obama was not electable. The campaign also took a shot at Messina, who now heads his own firm, the Messina Group, and was hired in 2017 to represent former British Prime Minister Theresa May, the leader of the Conservative Party.

“If he spent less time in boardrooms courting CEOs, he would witness the incredible energy generated by Bernie’s campaign, which has more than 5 million individual donations — the all-time record for a presidential candidate at this point in the race,” Sanders communications director Mike Casca said in a statement to POLITICO. “But please do make sure Jim gives our best to his dear friend Theresa May.” (Messina said May won when he worked for her.)

A second senior member of Obama’s campaign team, who asked not to be identified, sounded a similar warning, arguing Sanders would not be able to help Democrats in states like Arizona and North Carolina if they’re to defeat Trump.

“My concern about Sanders would be just how low his ceiling may be,” the adviser said. “The argument Sanders would make is he can turn out tough-to-turn-out voters. While many are very progressive like the Sanders base, most aren’t, most aren’t connected to politics, they tend to be more moderate,” the adviser said. “I think it’s a falsehood that all the people not registering to turn out are looking for the most classically liberal candidates — that’s just not true.”

Sanders surged starting in late October after recovering from a heart attack and receiving a pivotal endorsement from New York Rep. Alexandria Ocasio-Cortez. For more than a week, Sanders has more directly attacked Joe Biden, the national top polling candidate, arguing the former vice president has too much “baggage” to win a general election. Sanders has argued the excitement behind his campaign and his steady dominance in grass-roots fundraising makes him the more electable candidate.