Everytown was founded in 2014 by Michael R. Bloomberg, the former mayor of New York, who put in $50 million of his own money in an effort to form a group that could rival the National Rifle Association. Erika Soto Lamb, a spokeswoman, said that the group did not speak for Mr. Bloomberg, who has not made an endorsement since deciding against a third-party run earlier this year but who has been highly critical of Mr. Trump. Ms. Lamb said she could not comment on Mr. Bloomberg’s intentions.

In addition to the endorsement, Everytown pledged to help Mrs. Clinton’s campaign get out the vote in California’s primary on Tuesday, the spokeswoman said, hosting phone banks and door-to-door campaigns that would start over the weekend. The group also claimed credit for Mrs. Clinton’s wardrobe on Thursday, as she wore an orange shirt during her foreign policy speech as part of the group’s “Wear Orange” awareness campaign. Though it said it had no plans at the moment to begin advertising campaigns, it has done so in support of candidates and ballot initiatives.

Gun control has played a large role in the campaign so far, and Mrs. Clinton has frequently addressed the issue, seeking to draw clear distinctions between her positions and those of Mr. Sanders. Mr. Trump, for his part, has accused Mrs. Clinton of wanting to “abolish the Second Amendment.”

Mrs. Clinton has also run numerous ads on the subject. Before the New York and Connecticut primaries, the campaign ran one that featured the daughter of the principal of Sandy Hook Elementary School, who was killed in the mass shooting there in 2012. “No one is fighting harder to reform our gun laws than Hillary Clinton,” the daughter said in the ad.

Before the Iowa primary, Mrs. Clinton ran a broadcast ad speaking directly to the camera saying, “Either we stand with the gun lobby or we join the president and stand up to them,” an unnamed jab at Mr. Sanders. On the campaign trail, she took her attacks further, saying Mr. Sanders had “voted with the N.R.A.” and against some gun control bills.