Gun control was supposed to be the issue of 2018.

Yet it has receded so far into the background of the midterm congressional elections that Everytown for Gun Safety, a major player in gun control, is spending its money on ads covering abortion, health care and the Republican tax bill — but nary a mention of assault rifles or bump stocks.

There are new voices on the electoral playing field, including students from the Parkland, Florida, high school shooting in February that left 17 people dead.

But the cable news cameras that helped give the students a platform have turned elsewhere, leaving the activists struggling for attention in an election season crowded with the politics of the Supreme Court, President Trump and international crises.

“Perhaps the gun issue has waned a bit in the absence of highly publicized mass shootings in the past few months. The passion to enact gun reform is usually heightened after a mass shooting and then quickly dissipates,” said Adam Winkler, a professor at the University of California, Los Angeles, and the author of “Gunfight: The Battle Over the Right to Bear Arms in America.”

Everytown and its affiliate Moms Demand Action for Gun Sense in America did not respond to repeated requests for comment on the campaigns.

March for Our Lives, the student-led movement that sprang from the Parkland shooting, has attempted to keep the momentum with its #TurnoutTuesday series, which hosts events in cities across the country every Tuesday until the Nov. 6 elections.

Everytown has spent $1.9 million this election cycle, up from $386,922 in the 2014 midterms. But the spending was across a vast number of races, with an average of $861 spent per candidate, according to campaign finance data compiled by the Center for Responsive Politics.

A separate Everytown fund spent $1.9 million on a single candidate, Democrat Lucy McBath, a gun control crusader running in Georgia’s 6th Congressional District in the Atlanta suburbs.

Ms. McBath, whose son Jordan Davis was fatally shot when he was 17, has kept the gun issue front and center in her campaign.

Giffords PAC, the organization of former Rep. Gabby Giffords, who was shot in the head in 2011, also is hitting roughly a dozen Republican House candidates for siding with the National Rifle Association and defending gun rights.

In some cases, the pullback from gun control by Democratic candidates and their allies looks like a political calculation, although the move bucks their claim that stricter gun laws have become a “90 percent issue” among Americans.

In Missouri, a hidden-camera video caught Sen. Claire McCaskill boasting about her support of banning high-capacity firearm magazines. It raised eyebrows not because it was an unusual stance for the two-term Democrat but because she is talking about it during her re-election campaign.

In public, Ms. McCaskill has avoided mentioning gun laws as she fights for re-election in a pro-Trump state.

She insisted there was more than enough support in the Senate to push through universal background checks and bans on bump stocks, assault weapons and high-capacity magazines if Senate Majority Leader Mitch McConnell, Kentucky Republican, got out of the way.

“Well, if we elect enough Democrats, we’ll get some gun safety stuff done. They won’t let us vote on it. We’ve got 60 votes for a number of measures that would help with gun safety, but McConnell won’t let them come to the floor,” she said in the hidden-camera report by Project Veritas.

McCaskill campaign staffer Rob Mills told the undercover reporter that the senator avoids the topic in the race “because she has a bunch of Republican voters” in the state.

Her Republican challenger, Josh Hawley, regularly evokes the Second Amendment on the stump and warns that Ms. McCaskill threatens gun rights. It’s a winning issue for him even if it turns off liberal voters.

Democratic strategist Brad Bannon said that stopping gun violence was still a major issue for the party’s base.

He pointed to a recent Pew Research survey that found 81 percent of voters who support Democrats rank gun violence as a “very big” problem facing the country.

Just 25 percent of voters who back Republicans felt that way.

“An overwhelming majority of Americans and police chiefs favor a ban on high-capacity magazines, so McCaskill is on safe ground there,” Mr. Bannon said. “In a close election, it’s a great way to mobilize urban and suburban voters in St. Louis and Kansas City. House Democratic candidates are more likely to discuss guns because they aren’t running in rural areas like the Democratic Senate candidates.”