"Extending it [New START] has bipartisan support in Congress," Merkley said via Twitter on Friday.
New START expires in February 2021, but the Trump administration has said there is sufficient amount of time to determine whether or not it will extend the treaty.
Merkley accused President Donald Trump for putting the treaty in jeopardy.
“The New START Treaty has successfully kept the United States and Russia out of a modern-day nuclear arms race,” Merkley said.
The Senator from Oregon also warned that politicians in Washington must not risk unleashing a new Cold War.
New START is the last remaining arms control treaty in force between Russia and the United States. Signed in 2010, the agreement stipulates that the number of strategic nuclear missiles launchers must be cut by half and limits the number of deployed strategic nuclear warheads to 1,550.
Russia has repeatedly stated its readiness to extend the New START treaty without any preconditions, but the United States continues to be undecided about an extension.