In the midst of an intensifying debate over the role of so-called superdelegates in the Democratic presidential primary and their outsized support for Hillary Clinton over Bernie Sanders, the Vermont senator has picked up another superdelegate and, most remarkably, his first endorsement from a sitting U.S. senator.

Oregon's Jeff Merkley announced his endorsement of Sanders in a New York Times op-ed published Tuesday evening. The liberal Democrat, who is not up for reelection this year, is the first of the Senate's 44 Democrats to come out in support of Sanders, a Senate colleague. 40 Democratic senators have already publicly endorsed Clinton, herself a former New York senator.

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Sanders is, of course, an Independent who caucuses with the Democrats in the Senate, however, his fellow Independent in the caucus, Angus King of Maine, has yet to endorse him.

In his op-ed, Merkely explained that Sanders' positions on trade deals, fossil fuels and campaign finance reform helped cement his support.

"This really is all about the person who has the boldest, most fierce vision on the biggest issues facing America and the world," he said in an MSNBC interview Wednesday morning, arguing that Sanders “would do very well against” either Ted Cruz or Donald Trump.

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“Obviously the math is an uphill climb, but we've been surprised by what happens in campaigns time and again,” Merkley admitted when pressed on Sanders' path to the nomination, adding, “anything's possible.” Merkley also touched on Sanders' viability and the indelible impact of his campaign in his op-ed:

It has been noted that Bernie has an uphill battle ahead of him to win the Democratic nomination. But his leadership on these issues and his willingness to fearlessly stand up to the powers that be have galvanized a grass-roots movement. 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.

Merkley's endorsement Tuesday comes after Sanders nabbed another superdelegate on Monday, Minnesota Congressman Rick Nolan. As Politico noted, Minnesota is a sort of "superdelegate sweet spot" for Sanders, with three members of Congress — Nolan, and U.S. Reps. Keith Ellison and Collin Peterson -- publicly supporting Sanders. No other state has more than one U.S. House member backing Sanders.

Still, Sanders badly trails Clinton even with these additional endorsements. Sanders' superdelegate count of 40 is dwarfed by Clinton's 469 superdelegates. Luckily for Sanders, Oregon is expected to have 74 delegates, 61 will be so-called pledged delegates, and the most recent polling in the state shows Sanders with a commanding lead.

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Oregon's closed primary is May 17. The voter registration deadline is April 26.

Watch Merkely explain his endorsement of Sanders on MSNBC's "Morning Joe" below:

