A Democratic Super PAC is re-airing digital ads calling on two Republican senators to back an independent probe of Russian meddling in the 2016 election, following former F.B.I. Director James B. Comey’s firing Tuesday night.

Comey was leading the agency’s investigation into Russian interference and whether President Donald Trump’s campaign team colluded with the Russians. Trump fired Comey Monday night, purportedly for how he handled an investigation into Hillary Clinton, raising questions about the future of the Russia probe.

The surprise move prompted Senate Democrats to renew calls for a special prosecutor to lead the Justice Department’s investigation. The House and Senate Intelligence Committees are conducting their own probes, but some lawmakers have also renewed calls for an independent commission or special committee to investigate Russian meddling.

American Bridge, a Democratic-leaning Super PAC, is re-airing digital ads it launched in February, calling on Arizona Sen. Jeff Flake and Nevada Sen. Dean Heller, both GOP senators up for re-election in 2018, to support an independent commission.

“This unprecedented interference by Trump into the FBI’s Russia investigation makes the need for a truly independent investigation even clearer,” American Bridge President Jessica Mackler said in a statement. “Republicans like Dean Heller and Jeff Flake who stay silent now are guilty of moral cowardice that imperils our national security.”