Bernie SandersBernie SandersRussian interference reports rock Capitol Hill The Democratic nominee won't be democratically chosen Fox's Ingraham mocks DNC over Nevada voting malfunctions: 'Are we a Third World country?' MORE's lone supporter in the Senate is flipping to back Hillary ClintonHillary Diane Rodham ClintonRussian interference reports rock Capitol Hill Judge dismisses Nunes' lawsuit against Fusion GPS The Hill's Campaign Report: What to watch for in Nevada MORE for president. 

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Sen. Jeff MerkleyJeffrey (Jeff) Alan MerkleyInterest rate caps are popular — for good reason Overnight Energy: EPA to regulate 'forever chemicals' in drinking water | Trump budget calls for slashing funds for climate science centers | House Dems urge banks not to fund drilling in Arctic refuge Democratic senators criticize plan that could expand Arctic oil and gas development MORE (D-Ore.), the only senator to endorse Sanders, on Friday told CNN he would back Clinton just days after she became the party's presumptive nominee. 

"Certainly, I'm going to be supporting our nominee and our nominee is Secretary Clinton," Merkely told CNN. 

He also said Sen. Elizabeth WarrenElizabeth Ann WarrenThe Democratic nominee won't be democratically chosen Surging Sanders looks for decisive win in Nevada Bloomberg to do interview with Al Sharpton MORE (D-Mass.) would be a "wonderful vice presidential choice." 

Merkley earlier this week urged Sanders to rethink his campaign and back Clinton to unify the party before the general election, but he hadn't yet formally endorsed her until Friday. 

When Merkley endorsed Sanders in April, he said he was "boldly and fiercely addressing the biggest challenges facing our country."   

Clinton has won a number of big victories this week. She became the presumptive nominee after winning the California primary and has taken a commanding lead with the party's superdelegates, and she's been endorsed by President Obama, Vice President Biden and Warren.

Sanders, who met with Obama at the White House on Thursday, is continuing his campaign, but has dramatically softened his rhetoric in recent days, leading onlookers to believe he no longer plans to try to contest the Democratic convention in July.