A Democratic super PAC is targeting two GOP senators up for re-election in 2018, urging them to back an independent investigation into President Donald Trump’s ties to Russia. 

American Bridge 21st Century is launching a two-week digital ad campaign Thursday against Arizona Sen. Jeff Flake and Nevada Sen. Dean Heller, with plans to expand the campaign to other platforms. The ads are targeted to “engaged voters” of all parties and direct voters to a petition calling on the senators to support an independent investigation. 

It’s among the first paid efforts from a Democratic group to hold Republicans accountable for alleged connections between the president and Russia. The ads feature a black-and-white photo of each senator with yellow text saying, “Russia and Trump? Senator Flake, investigate.”

Democrats on Capitol Hill have called for an independent investigation. But they were divided over which body should lead the probe, with some asking that a separate bipartisan commission be established.

For now, Senate Democrats have settled on the Intelligence Committee leading the investigation. They’ve  laid out several demands, including asking Attorney General Jeff Sessions to recuse himself from any investigation.

American Bridge is now picking up the call for an independent investigation.

“With the stakes this high, Dean Heller and Jeff Flake need to support an independent investigation into Trump’s ties to Russia, and get the American people the answers they deserve,” American Bridge President Jessica Mackler said in a statement. 

Senate Republicans have maintained that the Intelligence Committee is capable of investigating and that a special committee is not needed. 

Flake has said he has confidence in Sessions to conduct an investigation.

“I would think he would have the independence to do it,” Flake said last week. 

The re-election of the first-term senator, who’s likely to face a primary threat, is rated Republican Favored by Inside Elections with Nathan L. Gonzales. 

Heller is the only Republican from a state Hillary Clinton won last fall facing re-election. His re-election is rated Lean Republican. 

Bridget Bowman contributed to this report.