“So many candidates for Congress, particularly women, are running on this issue — not just making it part of their platform and not just supporting it, but actually running on it,” said Shannon Watts, founder of Moms Demand Action for Gun Sense in America, the grass-roots arm of Everytown for Gun Safety. Everytown, which is largely funded by Michael Bloomberg, has endorsed 196 candidates this year in 36 states, and more than 40 volunteers for Moms Demand Action are running for office.

[On the road with the Parkland activists.]

One of them is Amber Gustafson, who was the group’s Iowa leader in the run-up to the 2016 election. A gun owner and former Republican, Ms. Gustafson is now challenging the majority leader of the Iowa Senate as a Democrat. (She went from Republican to independent in 2008 and registered as a Democrat in 2016, driven mainly by her views on health care and guns.) She said she discussed gun policy with every voter she visited.

“I say, ‘I’m a gun owner’ — and I always put my hand over my heart — ‘and I care about the Second Amendment, but I also want to make sure that guns do not fall into the hands of people who shouldn’t have them,’” she said.

She supports universal background checks and red-flag laws, which would allow the temporary removal of guns if a person’s relatives or law enforcement attest that they pose an immediate threat. She emphasizes that her opponent, State Senator Jack Whitver, supported a bill that would have dismantled Iowa’s permitting system for gun purchases.

Some of this year’s gun-control candidates are, like Ms. Gustafson, gun owners, who hope that will give them more credibility with some voters on the fence about restrictions. Mr. Crow, a hunter who served in Iraq and Afghanistan, likes to emphasize that he has “used weapons of war and had them used against me.”

“I’ve never needed an M4 or AR-15 or anything like it to deer, duck or rabbit hunt,” Mr. Crow said. “Nor would I need a weapon like that to defend my home or my family here in the United States.”

Tyler Sandberg, a spokesman for Mr. Coffman, the Republican incumbent, called bans on specific types of firearms or magazines “extreme.” He added that Mr. Coffman supported red-flag laws and had helped to write a bill that would offer federal grants for state red-flag programs, as well as a school security bill.