A Colorado-based progressive group said Wednesday that it’s putting up a billboard in Denver attacking Republican U.S. Sen. Cory Gardner’s record on gun-control legislation as Congress and the White House grapple with proposals to respond to gun violence.

“Protect kids not guns,” says the billboard, which is funded by Blue Rising PAC.

In addition, the billboard says, Gardner has benefited from $3.8 million in donations and independent expenditures on his behalf from the National Rifle Association.

“It is time Cory Gardner finds the courage to put the lives of Colorado’s children over the special interests of the gun lobby,” said Blue Rising PAC founders Dawn Reinfeld and Annette Moore in a statement. “He has an opportunity to be a leader on gun violence prevention legislation. However, his recent comments and voting record don’t leave us with much hope. Therefore, we are engaging our grassroots network to replace him in 2020.”

Gardner is seeking a second term next year in a race the Cook Political Report labels a toss-up. 

The billboard will be in place for the next 30 days in the 2400 block of South Broadway in Denver’s Rosedale neighborhood, a spokesman for the PAC said.

It’s the latest in a barrage of ads aimed at Gardner over his position on gun-control measures.

In August, the Center for Responsive Politics determined that Giffords, the organization founded by former U.S. Rep. Gabby Giffords of Arizona, spent just shy of $500,000 on a TV ad campaign calling on Gardner to support a universal background check bill passed earlier this yer by the Democratic-controlled House of Representatives.

Gardner was among a handful of Republican senators also targeted in August with $1 million in TV and digital ads paid for by Everytown for Gun Safety, a gun-control advocacy group funded by billionaire Michael Bloomberg.

Gardner’s Senate office didn’t respond to an inquiry about whether the lawmaker would support or oppose legislation enacting universal background checks, a federal “red-flag” bill or a limit on high-capacity ammunition magazines – three proposals among those under discussion as Congress returned from its month-long recess last week.

“I do not support gun control,” Gardner said at an event in Aspen following the mass shootings in El Paso, Texas, and Dayton, Ohio, in early August. He’s also said that any attempts to prevent gun violence must not infringe on constitutional rights.

Gardner points to a bill he’s sponsoring with U.S. Sen. Chuck Grassley of Iowa to increase funding for the Secret Service’s National Threat Assessment Center, which works on preventing violence.