A Democratic super PAC is launching a second wave of digital ads urging GOP Sens. Dean Heller Dean Arthur HellerOn The Trail: Democrats plan to hammer Trump on Social Security, Medicare Lobbying World Democrats spend big to put Senate in play MORE (Nev.) and Jeff Flake Jeffrey (Jeff) Lane FlakeRepublican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style Bush endorsing Biden? Don't hold your breath MORE (Ariz.) to support an independent investigation into President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE’s Russia ties.

American Bridge 21st Century is targeting two GOP senators up for reelection in 2018, pressing them to back an independent probe investigating Russian interference in the U.S. election and whether Trump allies communicated with Russian officials during the campaign.

“Donald Trump is putting our national security at risk, praising Putin and ignoring Russia’s aggressions,” one of the ads says. “We deserve to know why, but Sen. Dean Heller is turning a blind eye.”

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The digital spots will run in Arizona and Nevada for two weeks on YouTube and other video platforms. The ads call on voters to contact both GOP senators and tell them to sign a petition that urges Heller and Flake to back an independent investigation.

Heller and Flake are two of the most vulnerable Republican senators who are facing reelection races in 2018. Democrats will largely be on defense in the midterm elections, needing to protect 25 seats, including two seats held by Independents who caucus with the party.

"While Dean Heller and Jeff Flake ignore the questions surrounding Trump's ties to Russia, Donald Trump himself continues to put America's national security and the interests of the American people at risk,” said American Bridge president Jessica Mackler. “It's time to launch an independent investigation, senators."

Sen. Ben Cardin Benjamin (Ben) Louis CardinPPP application window closes after coronavirus talks deadlock Congress eyes tighter restrictions on next round of small business help Senate passes extension of application deadline for PPP small-business loans MORE (D-Md.), the leading Democrat on the Senate Foreign Relations Committee, and other Democrats have called for an independent commission to probe Russia’s interference.

The New York Times reported earlier this month that some of Trump's campaign staffers had repeated contact with members of Russian intelligence. But Trump and his team have denied those claims, with the president tweeting this week that reports about Russia are “fake news.”

The House Intelligence Committee is investigating Russian efforts to meddle in the 2016 election, as well as any ties between campaign officials and Moscow. Chairman Devin Nunes (R-Calif.) said Monday that he has seen no evidence that Trump officials had regular contact with Russian officials.