Demonstrators hold signs at the start of the March for Our Lives rally March 24, 2018 in Washington, DC.

During his 2010 congressional campaign, Democrat Tim Walz of Minnesota boasted about his A rating from the National Rifle Association. He opposed gun-control measures and snagged donations from pro-gun organizations. He also assured voters in his conservative district that, as a hunter and military veteran, he would never betray the state's grand tradition in firearms. Eight years – and tragedy – flipped his stance. In August, Walz won the gubernatorial primary on an aggressive gun-control platform. He endorsed a ban on assault weapons, pushed universal background checks and disavowed support from the NRA. He also donated the $18,000 the NRA gave him throughout his career to a veterans group. "I expect the NRA to spend millions trying to defeat me," he wrote in an op-ed following the February mass shooting that killed 17 at Marjory Stoneman Douglas High School in Parkland, Florida, which shifted his stance on guns. Walz is running statewide in Minnesota, which leans Democratic, while his 1st Congressional District leans Republican. There has been a substantial shift to the left in the Democratic Party in the 2018 elections. In the primaries, Democrats largely recalibrated their positions on gun measures, turned their backs to the NRA — and won. They now prepare for midterm battles in red-tinged states and districts against Republican pro-gun candidates and face a strong, newfound opposition from the NRA.

Democrat Tim Walz at his election night party on Aug. 14, 2018. in St Paul, Minnesota. Stephen Maturen | Getty Images

The Democratic Congressional Campaign Committee lists 63 Democratic House candidates on its "red to blue" list of challengers who could flip critical GOP seats in November. All but one campaigned to expand background checks for gun purchases, and none has NRA support. Democrats competing in conservative states historically have avoided making gun control a core plank in their campaigns for fear of alienating gun-rights advocates. But this year, many Democrats are emboldened by the groundswell of outrage over gun violence, opposition to the NRA and polls suggesting that Americans want stronger gun control. The party-wide shift on gun control in the primaries stems from mobilization following recent high-profile mass shootings in Las Vegas, Sutherland Springs, Texas, and Parkland, Florida. Democratic candidates this year were also fueled by grassroots pro-gun control organizations, which have seen an unprecedented spike in individual donations since 2016, according to public filings, and an uptick in volunteers since the Parkland shooting. Last week, Democrats touted what they called evidence of illegal coordination between the NRA and Montana GOP Senate candidate Matt Rosendale on an ad buy after a leaked audio recording was published Thursday by the Daily Beast. Democrats said the audio indicates that the nominee and the NRA violated federal laws prohibiting campaigns from working with interest groups on political advertising. Rosendale's campaign denied illegal activity. The NRA historically has backed more Republicans than Democrats, but the division is especially stark this year. During the 2012 elections, the NRA made campaign contributions to 30 Democratic House candidates. It supported 10 in the 2014 midterms and four in the 2016 presidential election. This year, the NRA is only backing three, according to public filings. Attacking the NRA, which claims a membership of nearly 5 million, is a political gamble for Democrats. "This doesn't usually happen," said Robert Spitzer, political scientist at the State University of New York at Cortland and author of five books on gun policy. "Political rallying of the Parkland students and their allies have gotten more Democrats to embrace a gun safety agenda, speak about it publicly and even brag about being opposed by the NRA. And it's prompted the NRA to heighten opposition against these candidates." In a 2018 primary that saw a major ideological divide in the Democratic Party between centrist incumbents and a wave of candidates running to their left, the unified shift toward pro-gun control measures represents another issue in which few Democrats now dissent, similar to the national party's stance on abortion rights and support for same-sex marriage.

Goodbye to the NRA

Arizona's Ann Kirkpatrick, a Democratic former congresswoman looking to make a comeback this year, was once a staunch NRA defender and praised it as a "civil liberties organization." But Kirkpatrick, who boasted an A rating from the NRA and condemned gun control measures in her 2010 campaign, flipped her stance following major shootings across the country.

Ann Kirkpatrick, Democratic candidate for Congress from Arizona. Bill Clark | CQ-Roll Call Group | Getty Images

She won her Democratic primary this year on the promise to ban assault weapons and enact universal background checks. "I've changed my mind," she said, further boasting that the organization has spent $150,000 over the years trying to defeat her. Rep. Tim Ryan of Ohio, who also won his Democratic primary this year, was out of step with the traditional left platform, boasting an A rating from the NRA. After the 2016 Pulse nightclub shooting in Orlando, Florida, that killed 49 people, Kirkpatrick and Ryan appeared at a House-floor sit-in for gun control. Ryan since donated the $20,000 he had received from the NRA to gun safety groups. "I am tired of offering up the same condolences again and again while Congress continues to sit on the sidelines," he said. Rep. Mike Thompson, D-Calif., who received over $50,000 in NRA contributions in past campaigns, is now chairman of the House Democrats' task force on gun violence prevention. He said in an email to CNBC that he promises to fight for strong background checks. His NRA grade fell from B+ in 2010 to F in 2016, a status he says he "has no plans to change." Gun control advocacy might play well in suburban swing districts that are essential for Democrats to gain control of the House in November, according to Leah Askarinam of nonpartisan elections site Inside Elections. In Montana, where gun ownership is deeply ingrained in the state's culture, Kathleen Williams won her Democratic primary and made prevention of gun massacres central to her platform. "If the NRA wants to give me an F for that, then I will proudly stand with all of you and say that F means 'fearless,'" she said. Beto O'Rourke, the Democratic challenger to Sen. Ted Cruz in Texas, advocates a new national universal background check law, opposes national gun-carry reciprocity and boasts an F rating from the NRA in a red state with a deeply embedded gun culture. O'Rourke is a serious outlier. The Texas delegation, since 1998, has received more NRA donations than any other state, and according to campaign finance data from the Center of Respective Politics, 27 out of 38 Texas delegation members received A or A+ ratings. All but one was Republican. "An NRA endorsement is toxic in the Democratic Party. But in this general election, Democrats appealing to swing state voters in rural districts might need that endorsement," said UCLA law professor Adam Winkler. The three NRA backed House Democrats are Rep. Collin Peterson of Minnesota; Rep. Henry Cuellar of Texas, who runs unopposed; and Rep. Sanford Bishop of Georgia. Cueller and Bishop are centrist Democrats in Clinton-won districts that represent rural populations with deeply ingrained gun cultures, and where firearms are a primary economic driver.

Aggressive new messaging

Democrats have put gun control front and center in their paid media advertising. Gun control platforms far outweigh pro-Second Amendment positions in 2018 compared with prior elections, Kantar Media data show. During the 2014 midterm campaign, over 30 percent of ads that mentioned guns or the NRA were run by Democrats, versus more than 60 percent in 2018, and virtually all now push for gun regulation. Pro-gun control ads run by Democrats have more than quadrupled in 2018 compared with the 2016 election. The NRA historically has more money than any other gun lobbying group. During the 2016 election, the Center for Responsive Politics estimated that the NRA and affiliates spent a record $54 million to secure GOP control of the White House and at least $30 million to elect President Donald Trump.

Rep. Beto O'Rourke, D-Texas Bill Clark | CQ Roll Call | Getty Images

But pro-gun control lobbies are countering the NRA's war chest, thanks in big part to former New York Mayor Michael Bloomberg, who launched Everytown for Gun Safety. Bloomberg also largely funds Moms Demand Action, which was launched following the Sandy Hook Elementary School school shooting in Newtown, Connecticut. Bloomberg has been pumping millions into Everytown, which has seen an increase in 265,000 individual donors in the past two years. It is unclear whether the pro-gun control Democrats' success in the primaries will translate to the midterms, and experts say it depends largely on activist voter turnout. There's some evidence that the gun-control movement could be a political force this November. An MSN poll just after the February massacre found that 70 percent of Americans support stricter laws on assault weapons – 87 percent of Democrats and 52 percent of Republicans, which is the highest ever in the 10-year history of the poll. And in April, a Washington Post-ABC News poll found that 53 percent of Americans think the gun-control movement after Parkland will be a lasting one, and 57 percent think new gun laws should be politicians' priority over protecting gun rights. In June, a Wall Street Journal/NBC News poll found that Democratic voters rated gun control as the second most important issue following health care, while Republicans rated it fifth, behind the economy, support for the president, immigration and taxes. A Harvard University Institute of Politics poll also found that 64 percent of 18- to 29-year-olds favor stricter gun laws, a 15-point jump since 2013. But it's unclear if these younger voters will show up at the ballot box in November. A recent analysis of voter registration data from Aristotle Inc. finds hardly any change in the overall share of registered voters ages 18 to 29 since the Parkland shootings. Stagnant voter registration, combined with low enthusiasm among younger voters and a history of low turnout in the midterm elections, could mean trouble for pro-gun control Democrats. But a countervailing reading by TargetSmart, a political analytics firm, found that registration rates for voters ages 18 to 29 increased in battleground states following the Parkland shooting. Said Winkler: "The Parkland shooting reignited passions for more gun regulation, but the question is, 'Will that movement lead to political transformation in November?'"

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