Bernie SandersBernard (Bernie) SandersOvernight Health Care: Drug industry nervous about Grassley | CDC warns public not to eat romaine lettuce | Sanders unveils new drug pricing bill Sanders and Khanna have a plan to lower your drug prices 2020 Democrats challenge Trump's use of troops at Mexico border MORE's lone supporter in the Senate is flipping to back Hillary ClintonHillary Diane Rodham ClintonWatergate’s John Dean: Nixon would tell Trump 'he's going too far' Senate Homeland Security chairman requests briefing on Ivanka Trump emails House GOP to hold hearing into DOJ’s probe of Clinton Foundation MORE for president. 

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Sen. Jeff MerkleyJeffrey (Jeff) Alan Merkley2020 Democrats challenge Trump's use of troops at Mexico border US detaining highest number of migrants in 17 years: report Warren, 2020 Dems target private immigration detention center operators 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 WarrenOn The Money: Senate banking panel showcases 2020 Dems | Koch groups urge Congress not to renew tax breaks | Dow down nearly 400 | Cuomo defends Amazon HQ2 deal Election Countdown: Florida fight ends with Scott, DeSantis wins | Dems see Sunbelt in play for 2020 | Trump to campaign in Mississippi ahead of runoff | GOP wipeout in Orange County | Ortiz Jones concedes in Texas House race Sanders on 2020 White House bid: 'We're looking at it' 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.