Manchester, New Hampshire (CNN) Former Vice President Joe Biden said Monday that Sen. Bernie Sanders' democratic socialist label would hurt Democrats' chances of winning control of the Senate in the general election if the Vermont independent is the party's presidential nominee.

In an interview with CNN's Don Lemon on "CNN Tonight" in New Hampshire on Monday, Biden bragged that Democratic candidates in red states had asked him to campaign with them in the 2018 midterm elections.

"Are they inviting him to speak?" Biden said of Sanders in the interview.

"Bernie's a great guy, but it's his self-definition," he said. "But the question is who can help us win back the Senate most?"

Biden's comments come as the Democratic field battles over which candidate is the most electable in a general-election matchup with President Donald Trump. Sanders and his campaign have said Biden's history of backing free-trade agreements and adjustments to entitlement programs would make him vulnerable to attacks from Trump; Biden has responded that Sanders' progressive platform of "Medicare for All" is politically unrealistic and would alienate voters in swing states.

"Are you going to walk into any of those states we have to win, like Florida, and other places that we're going to win, in Georgia and North Carolina," and embrace a Democratic nominee who describes himself as a democratic socialist, Biden asked. "What do you think Trump is going to do with that?"

In the interview, Biden -- who has polled well behind Sanders and former South Bend, Indiana, Mayor Pete Buttigieg in New Hampshire -- admitted he faced an "uphill race here."

He attributed the challenge to running against two candidates, Sanders and Massachusetts Sen. Elizabeth Warren,from neighboring states.

"I don't get a sense that there's any of that kind of panic. I was on the phone today with the South Carolina team; they feel good, I feel good," Biden said, nodding to the fourth state to vote, after Nevada.

And he said no Democratic contender can win the nomination without black voters backing them.

"The only people that have ever won are people who have overwhelming support in the African American community," he said.

Biden argued that he'll be best positioned to win in swing states against Trump. "The next nominee is going to have to be able to win in Pennsylvania and Michigan and Arizona and places that ... we didn't win last time," he said.

Biden also told Lemon he agrees with James Carville, the Democratic operative and former top Bill Clinton aide who in recent days has lambasted Democrats for embracing policies he's called out of touch with the voters the party will need in a general election.

"I think he's absolutely right about African Americans. I also think he's right about white middle class folks and working class folks. They want to know what you're doing. They want authenticity. Tell me what you're going to do, how it's going to work, how are we going to pay for it, who's going to pay for it. And tell me whether it's real. Can you get it done?" Biden said. "And that's the place where I think I'm in the strongest position. I've been able to bring people together, I've been able to get things done -- big things done."

This story has been updated with additional quotes from the interview.