SACRAMENTO, Calif. — A gun control initiative placed on the November ballot by California Lt. Gov. Gavin Newsom in the wake of several high-profile mass shootings is leading by a 2-to-1 margin with strong support from liberals who live on the coast, a new poll shows.

Proposition 63 would force ammunition purchasers to undergo background checks and outlaw possession of high-capacity ammunition magazines. It would do little to affect a package of legislation that Gov. Jerry Brown signed in July that already tightens rules for firearms owners. But that doesn’t seem to matter to voters.

The new Field/IGS Poll found 60 percent of likely voters surveyed the week of Sept. 7 support the initiative while 30 percent say they’re opposed. Only 10 percent reported being undecided. The poll also shows a similar share of likely voters favor Proposition 64, which would legalize marijuana for adult use. A poll on Wednesday from the Public Policy Institute of California also showed 60 percent of likely voters support legalizing marijuana.

Supporters of the gun control initiative applauded the poll results and said they’re proof of what Newsom has been saying since he launched the Safety for All campaign late last year: that voters deserve a chance to take on the gun lobby and endorse sensible rules that will make Californians safer. Last week, a USC Dornsife/Los Angeles Times poll found 64 percent of voters supported the measure.

“People are ready to stand up to the National Rifle Association and take bold action to reduce gun violence,” said Dan Newman, a spokesman for the Safety for All campaign.

Gun rights advocates dismissed the poll results and warned supporters not to get too comfortable, noting that turnout this season will be unusually tough to predict and that a ballot as crowded as the one voters must navigate in November could yield some surprises.

Along with requiring background checks for bullet sales and banning possession of high-capacity ammunition magazines, Proposition 63 would also force licensed vendors to report ammunition theft within 48 hours, make theft of a firearm a felony and create a new court process to ensure that firearms are surrendered by people upon conviction of serious crimes..