"We call on you to cancel all existing (Pentagon) contracts with Rosoboronexport, as well as any plans for future deals, and impose sanctions to ban contracts with any company that cooperates with Rosoboronexport on military programs," Sen. John Cornyn of Texas and Sen. Dan Coats of Indiana wrote Obama.
Both lawmakers are Republican. At least seven other senators were expected to sign the letter, a congressional aide said.
The Pentagon has paid Rosoboronexport over $1 billion for Russian-made Mi-17 helicopters, which the United States is providing to Afghan security forces.
Obama has imposed financial sanctions on members of Russian President Vladimir Putin's inner circle, and held out the prospect of sectoral sanctions - including in the defense sector - if Russia makes further military moves against Ukraine.
"Rosoboronexport is an arm of the Russian government and a powerful instrument of Vladimir Putin's increasingly belligerent foreign policy, and it handles more than 80 percent of Russia's weapons exports," the senators wrote.
Coats had previously offered an amendment to a Ukraine aid bill that would have cut off all U.S. business with Rosoboronexport; the amendment has not been acted upon.
The White House had no immediate comment.
In response to sustained criticism in Washington of the helicopter deals, Rosoboronexport's general director, Anatoly Isaykin, told Reuters in November that the contracts are "completely transparent" and "most acceptable from the perspective of price-to-value for the Defense Department."