Gun control advocates quickly urged Congress to act after the worst mass shooting in U.S. history. But the real fight may be on the campaign trail.

Many of them suggest that even after a gunman opened fire on thousands of concertgoers in Las Vegas on Sunday night — killing nearly 60 people and injuring more than 500 — it’s unlikely that the Republican-controlled House and Senate will act.

Those include Democratic Sen. Christopher S. Murphy of Connecticut, where 20 children and six adults at the Sandy Hook Elementary School in Newtown were killed in a mass shooting in 2012. Murphy has become a leading voice for preventing gun violence.

“We have a political movement that’s 20 years behind the gun lobby’s political movement,” Murphy said Monday. “So our effort has to be much more focused outside this building than inside this building.”

But the gun control movement still faces a number of challenges, including being outspent on the campaign trail, and trying to keep candidates and voters talking about gun violence long after coverage of mass shootings fades away.

“Elections matter”

Murphy launched his own “Fund to Prevent Gun Violence” last year, using his email list to direct campaign donations to six candidates he’s endorsed. A Murphy aide said the fund tracked the donations through ActBlue and estimated it raised about $190,000.

Former Arizona Rep. Gabrielle Giffords, who was shot in 2011 at a constituent event in Tucson, also held a press conference Monday with her husband, former astronaut Mark Kelly. They urged Congress to pass stricter gun laws.

“Who we elect matters,” Kelly said. “Don’t let anyone tell you not to talk about politics when we talk about guns.”

Giffords and Kelly launched their own group in 2013, Americans for Responsible Solutions PAC, to support candidates who back gun safety laws.

The group backed 25 House candidates and at least nine Senate candidates last cycle. Its independent expenditure arm spent over $2.7 million on a handful of races.

Everytown Action Fund, another gun control group, focused its spending in 2016 against two Senate Republicans: Kelly Ayotte of New Hampshire, who lost her race, and Ron Johnson of Wisconsin, who won.

The Pride Fund to End Gun Violence was launched days after the killing of 49 people at the Pulse nightclub in Orlando, Florida — which until Sunday held the title of the worst mass shooting in American history.

“What is the number one way that we can create change in this important fight for gun reform?” Pride Fund’s Executive Director Jason Lindsay said the group’s founders asked themselves. “What we determined was elections matter and the funding behind campaigns matter.”

The group was the first to endorse Democrat Stephanie Murphy against GOP Rep. John L. Mica, who represented part of Orlando. Lindsay said the organization launched a digital campaign and mobilized hundreds of volunteers in the Orlando area to support Murphy, who ended up unseating the 12-term incumbent.

Lindsay said the group, which focuses on gun violence but also backs candidates who support LGBT issues, plans to defend the Florida Democrat in 2018 and mobilize its 45,000 activists in other congressional elections.

Spending imbalance

Such groups are being vastly outspent by gun rights groups such as the National Rifle Association.

Take independent expenditures, on which groups spend the bulk of their money. Gun rights groups spent nearly $22 million on congressional races in 2016, supporting or opposing nearly 150 candidates.

But independent expenditures by gun control groups totaled nearly $3 million and focused on supporting or opposing six candidates.

Kelly said a campaign finance overhaul is necessary to fix that imbalance. But in the meantime, some advocates still see progress.

“We only have the money to play in three Senate races per year right now,” Sen. Murphy said. “But the races we’re playing in, we’re winning.”

Murphy pointed to Ayotte’s defeat in New Hampshire, Democrat Catherine Cortez Masto winning in Nevada, and Republican Patrick J. Toomey winning in Pennsylvania.

The Pennsylvania race brings up another question for the movement: Should it be partisan?

“Our firm belief is that we should only be supporting Democrats because they’re the only ones that are standing with us,” said Lindsay of the Pride Fund.

Lindsay said Democrats are consistently willing to pass gun safety laws, so the focus should be on helping Democrats win the House and Senate.

But other groups such as Giffords and Kelly’s Americans for Responsible Solutions backed Toomey last year because he worked on background check legislation after the 2012 Sandy Hook shooting.

Murphy said the effort should be bipartisan.

“We need to make Republicans who are on the wrong side pay and we need to reward Republicans who are on the right side,” he said.

Positive signs

As advocates grapple with raising resources and whether they should support one party, or two, they do see a general uptick in candidates who at least talk about gun violence.

Chelsea Parsons, who focuses on guns and crime policy at the liberal-leaning Center for American Progress, pointed out that voters largely support gun control laws.

“When candidates embrace those issues, really they are catching up to where their voters are,” Parsons said.

Gun policy ranked as the fifth most important issue in the 2016 election, according to the Pew Research Center. In 2012, gun control was the 13th most important issue.

Mass shootings, such as those in Newtown and Orlando, spur even more activists to join the gun control movement, advocates say. 

“The unfortunate thing is every time one of these mass shootings happens, tens of thousands of people join the movement to pass commonsense, anti-gun violence laws,” Murphy said. “And eventually, we’ll have enough manpower to overcome the gun lobby.”

Sean McMinn contributed to this report.Correction Oct. 4, 2:01 p.m. | An earlier version of this story misstated the name of a gun safety advocacy group. It is the Pride Fund to End Gun Violence.