Hillary Rodham Clinton told a longtime supporter and donor recently that she understands her friends are frustrated and worried by her slide in the polls. But the solution, she said, is not to attack the challenger who is surging as she slumps.

“I am not going to start to take shots at Bernie Sanders,” Clinton said, according to a person familiar with the exchange.

Clinton has held her fire for months as Sanders, the independent senator from Vermont who is seeking the Democratic presidential nomination, first nibbled and then devoured her once-commanding leads in New Hampshire and Iowa. She attacks Republicans daily but studiously avoids mentioning Sanders by name.

With the first Democratic debate slightly more than two weeks away, Clinton is batting away suggestions from antsy supporters that she take on Sanders and his unabashedly far-left views. She insists, people who have spoken to her recently said, that time will prove her right.

But her reticence is sowing unease among some backers, who argue she should confront the Sanders threat more directly. One backer, who asked not to be named in order to speak more freely, said Clinton risks looking overconfident or oblivious to the challenge Sanders poses.

“Bernie Sanders is not Barack Obama, for many reasons,” the backer said. “Still, a lot of us remember what it looked like last time,” a reference to Clinton’s 2008 run for the nomination.

Clinton is not ignoring Sanders completely. Last week, for example, she broke her long silence on the proposed Keystone XL pipeline — siding with Sanders against it. And she has been running hard against him for months in behind-the-scenes contests for campaign cash and Democratic delegates.

Appearing Sunday on NBC’s “Meet the Press,” Clinton was asked whether the zeal for San­ders among progressive voters may reflect a sense that he has long-standing positions such as opposing Keystone and supporting same-sex marriage that Clinton has only now adopted.

“Well, he can speak for himself,” Clinton said, without using Sanders’s name. “And I certainly respect his views. I can just tell you that I am not someone who, you know, stakes out a position and holds it regardless of the evidence or regardless of the way that I perceive what’s happening in the world around me.”

The hands-off strategy with Sanders is a bet that Clinton has the policy credentials and organizational muscle to withstand the senator’s challenge without angering his supporters overmuch. Polling suggests that most current Sanders supporters would support Clinton in the general election if she is the nominee, although the potential entry of Vice President Biden into the race could skew that calculation.

“I have five words of advice for the campaign,” longtime Clinton backer Paul Begala said when asked whether it’s time to start attacking Sanders. “No, no, no, no and no.”

Sanders’s positions may be to Clinton’s left in many cases, but none is as divisive or alienating as the far-right views of Donald Trump on the Republican side, Begala said.

“He’s raising issues we can sell in 2016,” Begala said of Sanders. “Donald Trump is running on a set of issues that make it next to impossible that Republicans can win in 2016. Anti-immigrant, anti-women. He’s driving them off the cliff.”

Clinton has not given a green light for supporters or surrogates to turn negative against Sanders, several people familiar with campaign outreach to backers said. Some Clinton backers have nonetheless quietly begun suggesting that Sanders is too liberal and his ideas too expensive and unworkable. A version of that argument is the crux of the comparison Clinton will eventually make herself, several supporters said.

Opposition research also surfaced this month from a super PAC that linked Sanders to the “extreme” views of Jeremy Corbyn, the new leader of Britain’s Labour Party.

Clinton’s campaign has worked closely with the super PAC, called Correct the Record.

Rather than hitting back at Clinton, Sanders sent a fundraising solicitation to his supporters telling them that he had been “pretty viciously” attacked. Within 48 hours, Sanders had raised more than $1.2 million through the solicitation, the campaign said.

“We are certainly prepared for the type of continuing attacks that we saw previously,” said Jeff Weaver, Sanders’s campaign manager. Clinton’s campaign has not publicly admonished the group to “cut out the low-brow, low-road attacks,” Weaver said. “If it were me, I would pick up the phone and tell them to cut it out.”

In a rare example of direct comparison, Clinton has called her college tuition program more “comprehensive” than one proposed by Sanders that would make tuition free at all public institutions, which leaves him open to criticism that he wants the government to pay to send rich students to college.

Clinton’s “compact” calls on families “to do their part” but seeks to dramatically reduce the debt that students incur.

“I am not going to give, you know, free college to wealthy kids,” Clinton said during a recent appearance before the Des Moines Register’s editorial board.

Asked Sunday on CBS’s “Face the Nation” about Clinton’s remark, Sanders said his college plan “is simple, it is straightforward. It exists in other countries, and in fact, 50 or 60 years ago, it used to exist in the United States of America.” He said the wealthy are likely to oppose many of his plans because they would be funded with tax hikes on the rich.

Sanders also said he takes Clinton at her word that she has no plans to attack him.

“Well, I certainly do, and I hope that will be the case,” he said, adding: “One of the reasons our campaign is doing well is we are focusing on the real issues that impact the middle class and working families of this country.”

Sanders noted that he has known Clinton for 25 years. “I respect her. I admire her. I’m not going to get into the business of attacking her,” he said.

Sanders aides said they expect further comparisons on issues, however, including the senator’s mixed record on gun control while representing a state with hunting traditions.

“I do anticipate that they’re going to try to draw contrasts on issues where they perceive they have differences,” said Tad Devine, a veteran Democratic strategist who is advising Sanders. “I think there’s going to be more of a push-back. I don’t know if it will get up to the candidate level before the debates, but I think we’re going to see more of it.”

Already, a super PAC backing another Democratic candidate, former Maryland governor Martin O’Malley, has gone after San­ders for his mixed record on guns.

In Web ads that debuted on the summer, the Generation Forward PAC highlighted several votes Sanders had taken over the years, including one in 1993 against the landmark Brady Bill, which mandated federal background checks on firearms purchasers. He has since voted for some gun-control measures more broadly supported by Democrats.

Matea Gold contributed to this report.