Sanders told MSNBC's Rachel Maddow that Judge Merrick Garland is "probably not the most progressive pick that he could have made."

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"But I will strongly support the president's selection of Judge Garland," he said.

President Obama nominated Merrick Garland, chief judge of the D.C. Court of Appeals, on Wednesday. Garland is viewed as a moderate judge who received substantial support from Republicans when he was confirmed. But the GOP has stood firm in its stance against holding a hearing on any Obama nominee.

"The idea that the president should not be able to make a nomination is totally absurd. I will do everything I can to see that there is hearings and a vote takes place and that Garland becomes seated on the Supreme Court," Sanders said.

Still, the Vermont senator said he would ask Obama to withdraw his nomination if he becomes president.

"I'm 100 percent prepared to support Judge Garland. I think he's clearly very knowledgable and can serve ably on the Supreme Court," he said. "[But] I think there are some more progressive judges out there."

Sanders said his litmus test for a potential nominee is that the justice be "loud and clear" in overturning the Supreme Court's decision in Citizens United v. FEC, which opened the door for independent political expenditures by nonprofit corporations. Critics of the decision say it created the current political climate in which outside groups spend millions on political campaigns with which they are not formally associated.

"I am very worried about the future of American democracy," he said. "That is my litmus test and that's what I would insist on."