Sen. Bernie Sanders of Vermont is preparing new lines of attack against Hillary Rodham Clinton on trade, gun control, and even the controversy over her State Department email to use on Saturday at their next televised presidential debate, which the Sanders team regards as — if not a do-or-die moment — perhaps his best chance to slow her political momentum this fall.

Sanders prepped into the night on Thursday with a Senate aide playing Clinton at mock sessions in Burlington, Vt. Yet Democrats inside and outside his campaign said that Sanders may be limited — by his own moves and by hers — in stopping a resurgent Clinton, who has built double-digit leads over him in many opinion polls. They noted that Sanders still does not want to go on the attack first against Clinton at the debate, potentially depriving him of chances to raise doubts about her judgment of character.

Another obstacle for Sanders is that Clinton has already co-opted many of the liberal positions that he once owned, like opposition to the Keystone XL pipeline and Trans-Pacific Partnership trade deal.

Sanders has vulnerabilities, too, like congressional votes against some gun control bills, and the fact that he is not a Democrat, despite seeking the party’s nomination, but a democratic socialist.

Sanders has also boxed himself into a corner somewhat on a central issue: He memorably said at the last debate that he did not want to talk about Clinton’s "damn emails," then found himself shaking her outstretched hand after dismissing the issue.

In fact, Sanders believes it is fair game for him to talk at Saturday’s debate about the federal investigation into her use of a private email server as secretary of state, his advisers said. But he plans to discuss the issue only if he is asked about it, the advisers added, a caveat that seemed meant as a signal to the debate moderators, but that also reflected the vise in which Sanders has put himself by swearing off negative campaigning.

"He’s closely studying her past remarks, trying to get a greater understanding of her past and present positions, so he can make the strongest substantive case for their differences on issues and decision-making," said Tad Devine, a senior adviser to Sanders.

"At the last debate, when the Trans-Pacific trade deal came up, Bernie could have said, ‘Wait a minute,’ and really pressed her on once calling it the ‘gold standard’ of deals," Devine said. "He could have brought up how she said, about gun control policy during the 2008 presidential race, that what might work in New York isn’t going to work in Montana. Bernie will have a full understanding of her record."

While Devine insisted that Sanders’ candidacy was not riding on Saturday’s debate, noting that his television advertising campaign had only just begun, he also acknowledged the high stakes.

"He’s got to continue introducing himself and find ways to convince people to say they can see him as president," Devine said. "We’re not winning Clinton voters yet. But Saturday will be a chance for many voters to see him on a prolonged basis, and that’s a huge chance for us to make an impact."

Clinton has been on a roll since the Oct. 13 Democratic debate, with Vice President Joe Biden deciding not to enter the race, and with her well-received performances at a congressional hearing on the Benghazi, Libya, attack and at a high-profile Iowa Democratic Party dinner. She is hoping for another strong moment at Saturday’s nationally televised debate, which will be in Des Moines, Iowa, and is expected to have a wide viewership in the state, the home of the first-in-the-nation presidential caucuses.

One reason the debate is so important to Sanders is that he has described the Feb. 1 Iowa caucuses as must-win — yet Clinton is leading in Iowa by as much as 30 percentage points in some polls.

Sanders is treating the event as an organizational opportunity, using a watch party across the street from the Drake University debate setting as a chance to collect the names of potential supporters.

One adviser to Sanders suggested he needed to be better prepared to discuss gun control, presenting himself as a candidate who understands the subject from both sides rather than allowing himself to be pigeonholed as a defender of the National Rifle Association.

Democrats unaffiliated with either campaign said that Sanders needed to find a way to return to the message that helped galvanize his supporters in the first place: that he knows what needs to change in the country, and that he would adhere most faithfully to liberal values as the party’s nominee.

They also warned that he needed to be prepared for Clinton and the other Democratic contender, former Gov. Martin O’Malley of Maryland, to press him where he is vulnerable.

"Sanders needs to make the case for why his approach to politics is better for achieving progressive goals than Clinton, not attack her character," said Dan Pfeiffer, a former communications director for President Barack Obama. "On a more basic level, Sanders needs to be more prepared for the obvious barbs coming from the moderators and Clinton and O’Malley on issues like guns, socialism and his backtracking on emails."

Others suggested that Sanders should not raise the email issue at all.

David Axelrod, a former top adviser to Obama, said that he was unsure why Sanders had sought to put it back into play.

"His core strength, his core value, is honesty, consistency," said Axelrod, adding that Sanders had harmed himself by trying to resurrect the controversy as an issue in the primary contest, saying it was of interest mainly to Republicans and perhaps some independent voters.

"If he’s going to pick fights with her, it ought to be over substantive issues, or his argument that he arrived at some of these positions before she did, and that he’s more reliable to hold to them in the future," Axelrod said.

Yet when Sanders delivered a thinly veiled critique of Clinton in a closely watched Iowa Democratic gathering last month, suggesting that she was a poll-tested flip-flopper, it was seen as being at odds with his reputation for avoiding attacks.

"He has to decide if he’s going to continue down the road of distinguishing his record from the Clinton record," said Jeff Link, a Democratic strategist based in Iowa, who worked on Obama’s campaign, "or if he’s going to go back to the formula that was successful for him, which is talking about what he believes in and where he’s going to take the country."

Link noted that Sanders had gotten 40 percent in the polls by being himself. "I just don’t understand," he said. "If something’s working, why would you switch?"

Far from scripting Sanders with canned lines, his advisers said they were using the debate prep sessions to make sure that Sanders could be himself on stage — by making sure that he was as comfortable as possible with Clinton’s record so he could joust with her with ease.

His Senate chief of staff, Michaeleen Crowell, was playing Clinton in the mock debates, while another Sanders aide played O’Malley.

Devine also prepared a 30-minute video of highlights from the first Democratic debate — though he said that Sanders seemed disinclined to watch it.

"Bernie is not a guy who likes to sit around and watch video of himself and other people," Devine said. "I mean, he doesn’t watch a lot of TV generally."

But that was not much of a problem, he added: "The bigger vulnerability for Bernie is if we got him away from who he is, how he naturally speaks, how he connects with people. That’s what he needs to do to win."

© 2015 The New York Times Company