Bernie SandersBernie SandersOvernight Healthcare: Senate votes to begin ObamaCare repeal debate | McCain returns to vote | GOP floats scaled-down healthcare bill OPINION | Healthcare vote a political death wish for GOP in 2018 Senate parliamentarian: More parts of ObamaCare repeal will need 60 votes MORE's lone supporter in the Senate is flipping to back Hillary ClintonHillary Rodham ClintonTrump turns up heat on AG Sessions over recusal Mellman: Trump love? Overnight Cybersecurity: Senate Judiciary reportedly drops Manafort subpoena | Kushner meets with House Intel | House passes Russia sanctions deal | What to watch at 'hacker summer camp' MORE for president. 

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Sen. Jeff MerkleyJeff MerkleyOPINION | Shailene Woodley: US should run on renewable energy by 2050 Gore wishes Mikulski a happy birthday at 'Inconvenient Sequel' premiere Callista Gingrich touts Trump's commitment to environment despite Paris deal pullout 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 WarrenWarren: Entire country must speak up on GOP healthcare bill OPINION | Healthcare vote a political death wish for GOP in 2018 House votes to repeal consumer arbitration rule 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.