Hillary Clinton Hillary Diane Rodham ClintonBiden leads Trump by 36 points nationally among Latinos: poll Democratic super PAC to hit Trump in battleground states over coronavirus deaths Battle lines drawn on precedent in Supreme Court fight MORE’s supporters are attacking liberal rival Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE more forcefully than ever before.

It’s a tactic that has some Democrats shaking their heads, while allies of Sanders, the independent senator from Vermont, cite it as evidence that their candidate is gaining traction.

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“When your opponents and people who represent them wade into the conversation with attacks against you, for us it’s a recognition of the fact that something must be working,” said Tad Devine, a senior adviser to Sanders. “It’s a basic rule in politics that you don’t attack somebody if they are not doing well.”

Sen. Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE (D-Mo.), a Clinton backer, launched the sharpest attack yet on Sanders on Thursday morning. Appearing on MSNBC’s “Morning Joe,” McCaskill assailed him for having “an extreme message” and being “unrealistic.”

The Missouri senator also complained that the media were “not giving the same scrutiny to Bernie Sanders that they are giving to certainly Hillary Clinton and the other candidates.”

Earlier this month, another Clinton supporter, Rep. Luis Gutiérrez (D-Ill.), took the fight to Sanders on the issue of immigration.

“I don’t know if he likes immigrants, because he doesn't seem to talk about immigrants,” Gutiérrez told Larry King on his show “PoliticKING.”

Recent opinion polls have shown support for Sanders building, especially in Iowa and New Hampshire, the states that hold the first contests in the primary process.

The best result yet for Sanders came on Thursday evening, when a CNN/WMUR poll showed him closing to within 8 points of Clinton in New Hampshire.

But that result was not an outlier.

A Suffolk University poll put Sanders within 10 points of the former secretary of State in New Hampshire, drawing 31 percent backing to her 41 percent. A survey from Morning Consult put the deficit in the Granite State at 12 points. Polling from Bloomberg in Iowa gave Sanders 24 percent in that state to Clinton’s 50 percent.

None of those findings changes the fact that Clinton is considered an overwhelming favorite to win the Democratic nomination.

But they do point to a rapid advance for Sanders, whose support has more than doubled in New Hampshire since he launched his campaign a month ago. The Vermont senator has also drawn large crowds on the campaign trail. Almost 5,000 people showed up to hear him speak at the University of Denver last Saturday.

Even some Democrats who are broadly supportive of Clinton fear that attacking Sanders will only burnish his appeal.

“As candidates rise in the polls, there ... are ways to encourage scrutiny [of them],” said Chris Lehane, who worked in former President Clinton’s White House and on then-Vice President Al Gore’s 2000 presidential campaign. But he added that Clinton backers needed to be very wary of “creating more conflict — conflict that translates into more energy for him.”

Another Democrat strategist, speaking anonymously for fear of repercussions from the Clinton camp, put it more strongly.

“It’s kinda perplexing that Claire McCaskill would go after him in a pointed way,” the strategist said. “It makes no strategic sense. The best response to Sanders is silence. You are going to end up motivating his supporters to become more active, become more vocal — and that is not a good thing for the Clinton campaign.”

The extent to which the jabs from McCaskill had the imprimatur of the official Clinton campaign has become the subject of speculation among Democrats.

The anonymous strategist suggested that she might have been issuing her vigorous criticisms unprompted, perhaps as part of an effort to prove her loyalty to Team Clinton. McCaskill backed then-Sen. Barack Obama (D-Ill.) against Clinton in the bitter 2008 campaign for the White House.

But Devine asserted that “normally those people don’t go on those shows without getting a briefing and being directed in a certain way.” He sought to draw a contrast between such tactics and the Sanders campaign. “We have got to play a different game altogether, with direct contact with voters, a rejection of negative politics, a commitment to issues and substance,” he said.

A Clinton spokesman declined to comment for this story.

The 2016 Clinton campaign is widely seen, by supporters and detractors alike, as being desperate to avoid the errors of 2008, when Obama wrested the nomination away.

Looking back at the earlier campaign, some Clinton loyalists argue she should have gone negative on Obama earlier. But even in 2007, Obama was in a much more competitive position than Sanders is now. This time around, could Clinton’s team use too much force against Sanders, with counterproductive consequences?

“I think the Clinton people would have to say he’s not really a threat, but he’s starting to get a little traction,” said Tobe Berkovitz, a Boston University professor who specializes in political communications. “I don’t think going after Bernie is the best strategy because what you’re doing is giving him bundles of oxygen. On the other hand, if they ignore him, he can generate more and more buzz.”

The other dynamic that complicates Team Clinton’s pushback against Sanders is the degree to which the Democratic primary electorate has moved leftward. A few election cycles ago, establishment candidates such as Gore in 2000 or then-Sen. John Kerry (D-Mass.) in 2004 could assail their left-wing rivals as beyond the mainstream.

But criticisms of Sanders for backing, say, universal healthcare or a new Wall Street tax to pay for vast increases in education spending, would likely backfire, and undo Clinton’s efforts to energize progressives for the general election campaign ahead.

Insisting that the recent attacks are “not going to go anywhere,” Devine said, “Bernie Sanders hasn’t hid from his political identity. His ideas — like universal healthcare, for example? We are happy to debate those.”