"One of the things I'm very concerned about is the Supreme Court seat. The appointment of this seat rightly belongs in the hands of President Obama," the Oregon Democrat told CNN's Alisyn Camerota on "New Day."
"What the majority in the Senate has done is to basically steal that from one presidency and try to deliver it to another, which is going to greatly and profoundly affect the legitimacy of the Supreme Court. And that's doing major damage to an essential institution in our country's vision."
Obama nominated Merrick Garland to replace conservative icon Justice Antonin Scalia after the late justice's death earlier this year, but Republican congressional leaders said the spot should be filled by the next president. National exit polls reflect that 21% of voters said the Supreme Court was a major factor in how they voted.
Merkley said some of his fellow lawmakers are "privately deeply ashamed" by this "theft" that prevented Obama's appointment.
"The theft is under way," he said. "It's not quite complete until the transition of power takes place. I hope my colleagues who never really anticipated that they would succeed."
"And I know many of them are privately deeply ashamed of what they've been pushed into doing by their leadership, by the Koch brothers and their associates who really went in and said we want you to do something unprecedented in US History: Take a Supreme Court seat under our constitution, assigned to one president, and move it to another," he added.
But despite the recent presidential election, things do not have to end for Garland, he said.
"And so I'm hoping that people will recognize what damage is being done, there's still time to -- to hold a vote on Merrick Garland and save the institution," Merkley said.