"They are a campaign vision," Merkley, an Oregon Democrat, told CNN's Jake Tapper on "The Lead." "You can call them a promise to fight for the direction we need to go."
Merkley was responding to a longstanding criticism from Democrats and others that Sanders' proposals -- including free tuition at public universities and universal health care coverage -- are too "pie in the sky."
The New York Daily News blasted Sanders on its front page declaring him a "fantasist."
But Merkley argued that even presenting the goals he espouses makes them more likely to happen.
"By laying out that vision, the likelihood of taking steps in that direction become much more significant," he told Tapper.
Earlier Wednesday, Merkley called the Vermont independent's vision "very bold and powerful."
"Bernie Sanders is the individual who has put forward a very bold and powerful vision on the biggest issues facing America and the biggest issues in the world," Merkley told CNN Wednesday.
More senators might have endorsed Sanders had they believed earlier in the race that he would have been a competitive candidate, Merkley suggested.
"I think most senators took their positions very early on, at a time that I don't think very many people anticipated that Bernie Sanders was going to become a significant player in the presidential primary," Merkley said. "Perhaps if we were to turn the clock back, there'd be a lot more company."
Merkley said Sanders' positions on climate change and reforming the campaign finance system resonated with him.
Speaking with CNN's Jake Tapper on "The Lead" Wednesday afternoon, Merkley acknowledged the difficult road ahead for Sanders.
"There's no question it's an uphill battle," Merkley said of Sanders' attempts to sway superdelegates. "But if you look at campaigns in the past, sometimes events occur on the campaign in which that uphill battle becomes a little easier and, often, successful."
Earlier in the day, Merkley had called Sanders "a determined leader" in a New York Times op-ed
"Bernie is a determined leader in taking on the concentration of campaign cash from the mega-wealthy that is corrupting the vision of opportunity embedded in our Constitution," he wrote.
Merkley wrote that Sanders' primary rival, Hillary Clinton, would be "a strong and capable president" but said Sanders will "rethink" American politics and the economy.
"People know that we don't just need better policies, we need a wholesale rethinking of how our economy and our politics work, and for whom they work," he said.
Sanders' opposition to trade deals with nations that underpay their workers as well as his support for addressing climate change make him the ideal leader, Merkley said.
"It has been noted that Bernie has an uphill battle ahead of him to win the Democratic nomination," he wrote. "But his leadership on these issues and his willingness to fearlessly stand up to the powers that be have galvanized a grass-roots movement."
More than 80% of current Democratic senators have endorsed Clinton, including Senate Minority Leader Harry Reid