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When President Obama addressed Congress ‪on Tuesday night, Hillary Clinton’s campaign had an ad ready for the occasion. The digital spot, which also aired on cable television, showed Mrs. Clinton speaking to the camera and invoking Mr. Obama’s push for new gun control measures as a chance for voters to decide whether candidates stand with the president or against him.

Mrs. Clinton did not name Mr. Sanders, but she didn’t have to. Mrs. Clinton, who was such a prohibitive leader in polls that her presence in the race discouraged some Democratic elected officials from challenging her, finds herself in a tightening contest with Mr. Sanders, a self-described Democratic socialist and independent from Vermont.

There are clear shades of warlike flashbacks for Mrs. Clinton and her aides. Harking back to her 2008 days trying to fend off Mr. Obama, then a senator, in the primaries, Mrs. Clinton talked on Tuesday about the belief that a “magic wand” can be waved to fix all problems — a line she used then to paint Mr. Obama as naïve, and is using now similarly on Mr. Sanders.

Days earlier, Mr. Obama, in an Op-Ed article in The New York Times, took veiled jabs at Mr. Sanders as insufficiently committed to gun control efforts and indicated his support for the eventual Democratic nominee, whomever it may be, should not be assumed if they have substantive differences on guns.

The Brady Campaign to Prevent Gun Violence has endorsed Mrs. Clinton. So have the political arm of Planned Parenthood and former Representative Gabrielle Giffords of Arizona, a victim of gun violence. All of that support is being deployed to help Mrs. Clinton avoid losses in Iowa or New Hampshire. This sense of tightening could help motivate her supporters.

But at a moment of deep voter anger at institutions and the establishment, it is serving only to solidify enthusiasm for Mr. Sanders, a decades-long politician, as an outsider.

