Two years ago, when Democrat Chrissy Houlahan began running for Congress, people told the military veteran and former high school teacher to stay away from one topic:

Gun control.

"There was a caution to me … that it wasn't a quote-unquote winning issue," she recalled.

But she ignored the advice. And since she announced her candidacy, a rash of mass shootings, including the one in February in Parkland, Fla., have rocked the country. Supporting major gun-control policies, she has been endorsed by gun-safety groups and is favored to win a newly redrawn seat that in its last configuration has been long held by Republicans.

The shift Houlahan noticed in the Sixth District, which now covers Chester County and part of Berks County — suddenly there was "permission to talk" about gun issues, she said — is one that activists say is echoed nationwide. Gun control, once considered a third rail in American politics, has emerged as a prominent issue in races across the country, particularly in several key congressional districts.

The prospects for passing gun-control bills in Congress would improve immensely if Democrats capture the 23 seats they need to flip the House – which some polls suggest they might do – and gun-control platforms might improve their prospects.

For many Republicans, gun rights remain a strong issue — but a lower-profile one this time than in elections past. The National Rifle Association has spent more than $6 million so far, according to public records– significantly less than its double-digit spends in past elections and than the $20 million shelled out for the midterms by Everytown for Gun Safety.

The latest Gallup Poll showed that more than 60 percent of Americans favor stricter gun control. It's "one of the most defining issues for Democrats running for Congress and to retake the House this cycle," said Peter Ambler, executive director of the Giffords organization.

In Bucks County, Democrat Scott Wallace campaigned with the father of a Parkland victim. State Rep. Madeleine Dean, seeking a House seat in Montgomery County, released an ad Monday touting her efforts to promote anti-violence bills at the state level. In Dauphin County, House candidate George Scott burned a rifle in a primary campaign ad. In conservative-leaning western Pennsylvania, Democratic Rep. Conor Lamb, who both held a rifle and promoted background checks in his first campaign ads, is endorsed for reelection by gun-control groups.

A poll conducted for Giffords said that in some races, talking about gun control helped Democrats increase their leads. And both the Brady Campaign and CeaseFire PA said more candidates than ever have sought their endorsements this cycle.

Across the country, Democrats have intensified their efforts to embrace an issue that was once taboo. They have aired substantially more ads mentioning guns than Republicans, according to a Wall Street Journal analysis, indicating the party is banking on the issue to mobilize voters.

One of the most hotly contested House races pits Virginia Rep. Barbara Comstock, a Republican backed by the NRA, against Democrat Jennifer Wexton, who's been vocal on gun control and was the subject of Giffords' first political ad of the year — a $1 million buy. Running for Congress in Georgia is Lucy McBath, who began working for gun control after her teenage son, Jordan Davis, was shot in 2012 by a man who said Davis' music was too loud. Guns have likewise become an issue in some gubernatorial races, such as in NevadaFlorida, and Georgia.

"There's no question 2018 is stacking up as the year candidates are running on gun safety in order to win elections," said John Feinblatt, executive director of Everytown for Gun Safety. "Candidates know that this is mobilizing voters."

Some gun-rights candidates have remained outspoken, from Georgia gubernatorial hopeful Brian Kemp to Montanan Senate candidate Matt Rosendale to Rep. Keith Rothfus, vying for Pennsylvania's new 17th District seat against Lamb.

"The NRA supports me because I support the Second Amendment," Rothfus said in an Oct. 16 debate with Lamb in western Pennsylvania. "Before we start to criminalize the behavior of responsible, law-abiding citizens, we want to make sure that a law will have an impact."

In addition to spending, other signs point to gun control being the louder side this year: The NRA removed its long-noted archive of candidate grades from its website. Houlahan's opponent, Greg McCauley (who supports several of the gun-control measures Houlahan does while also supporting concealed carry reciprocity and opposing an assault weapons ban), took a pro-gun section off his web page early in the campaign. In 2014 and 2016, more gun-rights advertisements appeared than gun-control ads; now, that trend has reversed, according to a USA Today data analysis.

And a few Republicans, including Bucks County Rep. Brian Fitzpatrick, who has been endorsed by the three major gun-control groups, are breaking ranks on the issue — something advocates say is also a sign of change.

"You are seeing more candidates at the margin like Fitzpatrick who will come out and be champions of the cause," said Kris Brown, co-president of the Brady Campaign to Prevent Gun Violence. "I think this election cycle is a proof point that more will be doing that in the future."

Even in Pennsylvania, where a stark divide on guns continues, the state legislature this month passed a bill that tightened rules requiring convicted domestic abusers to relinquish their weapons — the first gun-control bill passed in the state in years. About half the states passed some form of gun control after Parkland. Washington state has an initiative on November's ballot that would give it some of the most stringent laws in the country.

Even if a majority of gun-control supporters is elected to the House, there's no guarantee legislation passed there would be approved by the Senate and signed by President Trump.

And a strong partisan divide remains on the issue, according to Pew research released last week. In that survey, only 18 percent of Americans said they'd expressed opinions about guns on social media in the last year and 3 percent said they'd attended a rally or protest about the issue.

Still, that survey said 57 percent of Americans support stricter gun laws, up 5 percent from last year. Advocates say progress in the midterms is a critical step on a longer road.

Just getting a vote in the House on background checks, which hasn't happened in more than a decade, would be "monumental," Brown said. (Last week, House Minority Leader Nancy Pelosi (D., Calif.) said passing a bill for universal background checks would be among the Democrats' first priorities.)

Many credit the activism by Parkland students for being the catalyst needed to cement gun control as a voter priority. Classmates of the 17 people killed have spoken out, encouraged young people to vote, and kept the issue in the news, advocates say, bolstering the work they began after the December 2012 shooting at Sandy Hook Elementary School.

"We'll know by Nov. 7 whether it really has changed or not," Brown said, "and I think we'll wake up and find that it has."