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.