Sen. Dean Heller Dean Arthur HellerOn The Trail: Democrats plan to hammer Trump on Social Security, Medicare Lobbying World Democrats spend big to put Senate in play MORE (Nev.), considered the most vulnerable Republican senator up for reelection next year, will be at the center of a new fight over gun control following the mass shooting in Las Vegas on Sunday that left at least 59 people dead.

Democrats and liberal activists plan to scrutinize Heller’s record on gun control and his co-sponsorship of a controversial bill to deregulate suppressors as they seek to win his Senate seat in 2018.

Critics are already zeroing in on Heller’s vote against popular legislation to expand background checks and limit ownership of assault rifles in 2013. They are also looking at his support for Senate Bill 59, which would ease regulations on suppressors, and Senate Bill 446, which would expand the right to carry concealed firearms.

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“When we’re done grieving, I think it’s time to bring up the issue that certain politics are killing people in the United States,” said Maria-Teresa Liebermann, the deputy director of Battle Born Progress, a progressive advocacy group based in Clark County, Nev.

“People not taking action on extended background checks, people constantly blocking any type of progress that we end gun violence,” she added.

Heller on Monday flew back to his home state, missing afternoon votes and leaving colleagues uncertain about his return. Sen. Catherine Cortez Masto (D-Nev.) also canceled events in Washington and flew back home Monday.

Heller spoke to Gov. Brian Sandoval (R) and state Attorney General Adam Paul Laxalt (R) early Monday morning, thanked police and first responders and urged people in Las Vegas to donate blood to help treat the more than 500 wounded or injured in the shooting.

“I will continue to monitor the situation as this horrific event unfolds. Lynne and I are praying for all of the victims and their families who are experiencing immense pain and grave, shocking loss that cannot be measured,” he said in a statement Monday, referring to his wife.

Heller joined most Republicans in voting in April of 2013 against bipartisan legislation sponsored by Sens. Pat Toomey Patrick (Pat) Joseph ToomeyAppeals court rules NSA's bulk phone data collection illegal Dunford withdraws from consideration to chair coronavirus oversight panel GOP senators push for quick, partial reopening of economy MORE (R-Pa.) and Joe Manchin Joseph (Joe) ManchinBiden promises Democratic senators help in battleground states Senate leaders quash talk of rank-and-file COVID-19 deal OVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' MORE (D-W.Va.) to expand background checks.

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A poll from October showed that 58 percent of respondents in Nevada supported expanded background checks, while only 32 percent opposed the idea.

Nevada voters passed a ballot initiative known as Question One last year that expanded background checks for private-party gun sales. Laxalt, however, has declined to implement the initiative because he says the FBI will not conduct the checks through the National Instant Criminal Background Check System.

“Gun violence has been a big issue here in Nevada,” said Liebermann. “It’s been addressed here locally and it’s not being enforced.”

The ballot initiative passed by a margin of 50.4 percent to 49.6.

Heller also opposed an amendment drafted by Sen. Dianne Feinstein Dianne Emiel FeinsteinMcConnell says Trump nominee to replace Ginsburg will get Senate vote Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Intensifying natural disasters do little to move needle on climate efforts MORE (D-Calif.) to regulate rifles with military styling and another sponsored by the late Sen. Frank Lautenberg (D-N.J.) to curb high-capacity magazines.

While Democrats think gun control could be a winning issue for them in some states, it also poses some political risks to the party.

Democrats are defending more than two-dozen seats in 2018, including 10 in states won by President Trump last year.

Sen. Heidi Heitkamp Mary (Heidi) Kathryn HeitkampCentrists, progressives rally around Harris pick for VP 70 former senators propose bipartisan caucus for incumbents Susan Collins set to play pivotal role in impeachment drama MORE (N.D.), one of those 10 Democrats, voted with Heller in 2013 against expanding background checks.

Most of the other nine Democrats, however, backed the measure sponsored by Manchin, who himself faces reelection next year in a state where Trump is popular.

Democratic Sens. Tammy Baldwin Tammy Suzanne BaldwinKeep teachers in the classroom Cher raised million for Biden campaign at LGBTQ-themed fundraiser Democrats seek balance in backing protests, condemning violence MORE (Wis.), Sherrod Brown Sherrod Campbell BrownBipartisan praise pours in after Ginsburg's death Emboldened Democrats haggle over 2021 agenda Hillicon Valley: Russia 'amplifying' concerns around mail-in voting to undermine election | Facebook and Twitter take steps to limit Trump remarks on voting | Facebook to block political ads ahead of election MORE (Ohio), Robert Casey Robert (Bob) Patrick CaseySecond GOP senator to quarantine after exposure to coronavirus GAO report finds brokers offered false info on coverage for pre-existing conditions Catholic group launches .7M campaign against Biden targeting swing-state voters MORE (Pa.), Joe Donnelly Joseph (Joe) Simon DonnellyEx-Sen. Joe Donnelly endorses Biden Lobbying world 70 former senators propose bipartisan caucus for incumbents MORE (Ind.), Claire McCaskill Claire Conner McCaskillDemocratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally Missouri county issues travel advisory for Lake of the Ozarks after Memorial Day parties MORE (Mo.), Bill Nelson Clarence (Bill) William NelsonDemocrats sound alarm on possible election chaos Trump, facing trouble in Florida, goes all in NASA names DC headquarters after agency's first Black female engineer Mary W. Jackson MORE (Fla.), Debbie Stabenow Deborah (Debbie) Ann StabenowOVERNIGHT ENERGY: Trump rollbacks could add 1.8 billion tons of greenhouse gas emissions over 15 years: analysis | Intensifying natural disasters do little to move needle on climate efforts | Experts warn wildfire smoke could worsen COVID-19 GAO report finds brokers offered false info on coverage for pre-existing conditions Democrats back away from quick reversal of Trump tax cuts MORE (Mich.) and Jon Tester Jonathan (Jon) TesterPence seeks to boost Daines in critical Montana Senate race This World Suicide Prevention Day, let's recommit to protecting the lives of our veterans Filibuster fight looms if Democrats retake Senate MORE (Mont.) all voted for the background checks legislation.

While Congress has made little progress in passing gun control bills in recent years, and even has appeared more inclined to loosen existing regulations, Democratic leaders think the politics of the issue are changing.

Democrats knocked former Sen. Kelly Ayotte Kelly Ann AyotteBottom line Bottom line Bottom Line MORE (R-N.H.) out of office last year by attacking votes she cast against expanded background checks in 2013 and 2016.

Four years ago, Senate Democratic Leader Charles Schumer Chuck SchumerPelosi orders Capitol flags at half-staff to honor Ginsburg Ginsburg in statement before her death said she wished not to be replaced until next president is sworn in Democrats call for NRA Foundation to be prohibited from receiving donations from federal employees MORE (N.Y.) said the country was on the verge of a “turning point” on gun control after the mass shooting at Sandy Hook Elementary School. He said data showed voters “even in redder states” want “common sense measures.”

“We should continue to take these legislators to task, and if they vote the wrong way, like Kelly Ayotte, they’re going to be held accountable for those votes,” said Christian Heyne, legislative director of the Coalition to Stop Gun Violence.

Heyne criticized Heller for what he called “a storied history of irresponsible votes and co-sponsorships,” citing his votes against gun control measures in 2013 and his co-sponsorship this year of bills deregulating suppressors and concealed firearms.

Senate Republican Whip John Cornyn John CornynBipartisan praise pours in after Ginsburg's death Chamber of Commerce endorses McSally for reelection Airline job cuts loom in battleground states MORE (Texas), a co-sponsor of the gun suppressor bill and the sponsor of the measure loosening restrictions on concealed weapons, said it was unseemly for some Democrats to be looking ahead to the political fight over gun control.

“Politicizing this terrible tragedy is beyond disgusting and I think we ought to wait a respectful period of time out of respect for the people who lost their lives or who have been injured before we get into the push and push of politics around here,” he said.

But Sen. Ed Markey Edward (Ed) John MarkeyDemocratic senator calls for eliminating filibuster, expanding Supreme Court if GOP fills vacancy McConnell says Trump nominee to replace Ginsburg will get Senate vote Massachusetts town clerk resigns after delays to primary vote count MORE (D-Mass.) pushed back on the idea that it was too soon.

“For anyone who says this debate is too soon, it’s already too late for at least 58 people in Las Vegas and hundreds of others who were wounded. We should not wait another day,” he said.