The former campaign manager for President Barack Obama believes Sen. Bernie Sanders is "the worst candidate" for the Democrats to select as their 2020 nominee.

Speaking Thursday on Morning Joe, Jim Messina acknowledged Sanders's rise in polling over the past month but warned that his brand of politics could spell disaster for Democrats in the general election.

“What do you make of Bernie Sanders leaping nearly double in the space of a month?” asked Morning Joe co-anchor Willie Geist.

“He’s beginning to consolidate the Left in his party, said Messina. “He has a relationship with these voters, he’s been campaigning with these voters for now six years, he knows New Hampshire voters really well, and people are beginning to come to him.”

The panel dissected Sen. Elizabeth Warren's flailing campaign before turning back to Sanders's surprise bump in a New Hampshire poll, released Thursday morning, showing the Vermont senator far ahead of the field in the early primary state.

"What is interesting and understandable, I think, about those numbers among millennials, is the devotion to Bernie Sanders is clearly understandable, in large part, I think, because of his consistency over the years on issues," said Morning Joe contributor Mike Barnicle. "They value consistency."

"Makes him unlike other, in their minds, politicians, and don’t underestimate the fact that Bernie, a lot of what he says, the messages, basically, 'You can live your life for free, we'll have free college tuition, we'll have free this, we’ll forgive your student debts,' it's a powerful message," he added.

"But in the general election, it's a message that I think is going to get him killed. I think he’s the worst candidate in a general election for exactly that reason," Messina countered.

"You think Bernie is the worst candidate?" asked Geist.

"I don’t think there’s a question about it," replied Messina. "I think it's very clear to me that these swing voters, that I care about, the Trump-Obama voters in Midwestern states, Bernie Sanders is not the candidate we need to beat Donald Trump in November."

Sanders's campaign has slammed media attacks against the candidate in the past, recently criticizing the December Democratic debate over a moderator's question, which appeared to assume Sanders was lying about a private 2018 meeting in which Warren claimed he said a woman could not win the presidency. His team called the moment "cringe-worthy."

[Read more: 2020 Democrats tiptoe around fraught history with Bernie Sanders]