Democrats running against Sen. Bernie Sanders are already making the argument that he would be a weaker general election candidate against President Trump because he is a democratic socialist - not a Democrat.

There's little question there's a path for Sanders to win the Democratic nomination.

Sanders, who caucuses with Democrats but serves in the Senate as an independent, raised $6 million in his first day on the campaign trail, underlining the seriousness of his candidacy.

But opponents of Sanders hope they can get voters to think twice about "feeling the Bern" if they have doubts about his ability to defeat Trump.

And they see the socialist argument as a good one.

"I think socialist is a word that someone who wants to beat Trump should consider carefully before embracing," said one senior adviser to a rival campaign.

It's not a new argument. Hillary Clinton and her supporters offered a similar cautionary tale during their bruising 2016 primary fight against Sanders.

But it could have more resonance in 2020 as Democrats try to find the person who can defeat Trump after their devastating disappointment in 2016.

Sanders allies argue there's little merit to the argument, and they say it won't work.

"He's clearly a Democrat and the party has grown enormously because of his efforts," said Larry Cohen, the chairman of Our Revolution, the political organization which spun out of Sanders's 2016 campaign. "And his real brand is that he doesn't just speak to Democrats. He speaks to literally millions of people that are unaffiliated. And he's really talking about what we should aim for and where we can build a consensus."

Bill Press, the talk show host who served as a surrogate for Sanders's 2016 campaign, added that the "democratic socialist" label will not hurt the candidate.

"That's who Bernie is, everyone knows it, and he does a good job defining what he means by a democratic socialist," Press said. "After he's done talking about a $15 minimum wage, 'Medicare for all,' and free college tuition, everybody wants to be a democratic socialist."

But this week, 2020 candidates - and those considering entering the race - have been actively drawing a distinction with Sanders and his brand.

At a campaign stop in New Hampshire earlier this week, Sen. Kamala Harris (D-Calif.) was the first to draw a line in the sand.

"The people of New Hampshire will tell me what's required to compete in New Hampshire, but I will tell you I am not a democratic socialist," Harris said after she was asked if she needed to be a democratic socialist to win the Granite State.

"I believe that what voters do want is they want to know that whoever is going to lead understands that in America today, not everyone has an equal opportunity and access to a path to success," she said before adding that the U.S. needs to "course correct" on the issue of income inequality.

At the same time, former Rep. Beto O'Rourke (D-Texas), who is deciding whether or not he'll run for president, also distanced himself from the label by declaring that he's a "capitalist."

"I don't see how we're able to meet any of the fundamental challenges that we have as a country without, in part, harnessing the power of the market," O'Rourke told reporters in El Paso, Texas.

At a CNN town hall on Monday, Sen. Amy Klobuchar (Minn.), a centrist Democrat, made it known that she thinks Sanders's policies are slightly unrealistic. When she was asked if she could get behind Medicare for all, she said, "It could be a possibility in the future."

"I'm just looking for something that will work now," she said, adding in a matter-of-fact way that she also was against free four-year college for all.

In his State of the Union address earlier this month, Trump previewed his campaign argument by taking a dig at socialism.

"We are born free, and we will stay free. Tonight, we renew our resolve that America will never be a socialist country," he said, as Republicans chanted "USA! USA!"

Republican operatives say they will continue to push the issue that the Democratic Party is being run by fringe candidates who they say push policies out of step with average Americans.

"Bernie Sanders is a self-avowed socialist who wants to double your taxes so the government can take over your health care," Michael Aherns, spokesman for the Republican National Committee, said in an email. "The vast majority of voters oppose his radical agenda, just like they are going to oppose all the 2020 Democrats who have rushed to embrace it."

At the same time, some Democrats cautioned the candidates to not be so label obsessed.

"2020 candidates are being framed by their degrees of separation from Sanders and his policies but they shouldn't fall into that trap," said Basil Smikle, a Democratic strategist who served as an aide to Hillary Clinton. "Voters aren't rejecting policies we're espousing. They're just rejecting labels and party affiliations to some degree."

"Democrats in their primaries will likely focus on who can reclaim much of the Obama coalition and be able to go toe-to-toe against Trump rather than be dragged into a long policy debate on socialist versus capitalist agendas," Smikle added. "That's a no-win scenario for us."