Senator 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.

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

Another obstacle for Mr. Sanders is that Mrs. 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.

Mr. 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.