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.