Story highlights Democratic lawmakers have pushed for the appointment of a special prosecutor into the Trump campaign's ties to Russia

Trump told NBC News he was thinking of "this Russia thing" when he decided to fire former FBI Director James Comey

Washington (CNN) Democratic Sens. Dick Durbin and Dianne Feinstein are calling on Deputy Attorney General Rod Rosenstein to resign if no special prosecutor is named to replace ousted FBI Director James Comey in the investigation into Russia's meddling in the 2016 US election.

"In an interview with NBC News, President (Donald) Trump admitted to firing FBI Director James Comey because of his investigation of the Trump campaign's Russian connections," Durbin, who is Senate Minority Whip, said in a statement. "That is dangerously close to obstruction of justice. This morning, the President tweeted a thinly veiled threat to Mr. Comey, which could be construed as threatening a witness in this investigation, which is another violation of federal law."

Feinstein, a California Democrat, echoed the Illinois Senator.

"I also support Senator Durbin's call for Deputy Attorney General Rod Rosenstein to resign if he is unwilling to appoint a special counsel," she said. "These investigations are far too important to risk disruption, delay or interference."

Their comments come as concern grows over the Trump campaign's ties to Russia. While White House officials -- including Vice President Mike Pence -- initially said Trump's decision to fire Comey came on the recommendation of Rosenstein, Trump offered a conflicting reason in an interview with NBC

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