A Democratic group devoted to getting big money out of politics launched a $1 million ad campaign Thursday targeting U.S. Rep. Mike Coffman for the millions the five-term Aurora Republican has raised from corporate interests in his runs for Congress.

“There are millions of reasons career politician Mike Coffman’s got to go. Special interests and corporate PACs spent $17 million to keep him in D.C.,” says a 30-second End Citizens United TV ad, set to air through Sept. 17.

The group, which endorsed Coffman’s Democratic challenger, Jason Crow, a year ago, is spending $645,000 on broadcast and cable ads, $215,000 on digital advertising and $127,000 on direct mail with the same message, a spokeswoman said.

“Over the past two years, we’ve conducted extensive research and polling on this and actually found that a money in politics message is an incredibly powerful issue with voters,” Anne Feldman, End Citizens United’s press secretary, told Colorado Politics. “Not only does it energize the base, but it moves independents and unaffiliated voters – many of whom voted for Trump and his message of draining the swamp.”

Like dozens of Democrats running nationwide this cycle, Crow has pledged to reject contributions from corporate PACs – a relatively cost-free move for a first-time candidate, critics point out, since big business doesn’t typically shovel money their way.

A poll commissioned by End Citizens United in February showed Crow leading Coffman by 5 points. But the Democrat’s lead increased to 10 points after voters were told Crow “is not taking a dime” from corporate PACs, while Coffman “has accepted $1.6 million from corporate PACs and just voted to give corporations a huge tax cut,” a reference to the GOP tax plan passed with Coffman’s support at the end of last year.

The campaign finance reform group has said it plans to spend $35 million targeting 20 Republican incumbents in the midterms. It spent what it called six figures on an ad hammering Coffman for his vote on the tax bill.

In December, ECU named Congressman Coffman to its Big Money 20 target list for the 2018 midterms. As part of the Big Money 20 campaign, ECU made a six-figure issue ad buy in January targeting Coffman for his vote on the tax bill. You can view the ad here.

Coffman’s campaign manager dismissed the ad campaign, raising an attack leveled by a primary opponent who charged Crow, a corporate attorney, wasn’t as squeaky clean as he portrayed himself.

“Another day, another Nancy Pelosi front group spending millions of dollars to beat Mike Coffman,” Tyler Sandberg told Colorado Politics. “As Levi Tillemann said, Jason Crow has been taking laundered corporate PAC money his whole campaign. Money laundering and financial crimes are something Jason Crow knows well, having defended corporate executives that embezzled employee pensions, ripped off hospitals and schools, and even an executive that defrauded veterans.”

The perennial battleground 6th Congressional District is one of two dozen nationwide won by Democrat Hillary Clinton but held by a Republican. The Cook Political Report lists it as a toss-up, though Roll Call this week moved Coffman up a notch on its list of the 10 most vulnerable House members.