Bernie Sanders' Sole Senate Endorser Says 'We Have Our Nominee' The senator is urging people to unite around the presumptive nominee.

 -- Bernie Sanders’ only Senate supporter, Sen. Jeff Merkley, said the Democratic Party now has made its choice, and as a result he is urging others to unite around that presumptive nominee, Hillary Clinton.

Merkley, of Oregon, said he believes Sanders will stay in the race until all Democratic voters have had a chance to make their preference known, which will happen after Washington D.C. holds its primary next Tuesday.

“He didn't make that successful ascent although he came very close,” Merkley said of Sanders, “but by enabling citizens to express themselves, this creates the foundation to now turn to them and say we need to come together.”

Merkley added that Sanders was already reaching out to the Clinton campaign and its supporters to ensure a smooth transition from the primary election to the general campaign.

“Sanders is already on the path to hold the conversations to start to bring the party together. He said we will unify at the convention, he said he'll do everything possible to make sure Donald Trump is not in the Oval Office,” he said. “He's setting up meetings with his supporters to make sure his ideas get carried forward.”

Merkley also believes it would be “very, very divisive and inappropriate” for Sanders to sway superdelegates away from Clinton.

While he thinks his path to the presidency has come to an end, Merkley said he was open to Sanders becoming Clinton’s running mate –- if that’s what the senator from Vermont wants.

“I would love to see a Vice President Sanders. I would love to have seen a President Sanders. But I think for Senator Sanders it has never been about holding an office. It has been about making America work for the American people,” Merkley said.

Rep. Peter Welch, D-Vermont, one of Sanders’ biggest boosters on Capitol Hill, acknowledged the writing on the wall for the Vermont senator in an interview Wednesday.

“We’re going to have a nominee and that’s going to be Hillary Clinton,” he said.

Welch hasn’t spoken to Sanders since the California primary, but doesn't believe Sanders is primarily interested in a position on Clinton's ticket.

He also suggested Sanders shouldn’t support calls to replace Rep. Debbie Wasserman Schultz, D-FL, as the leader of the Democratic National Committee, as some of his backers have suggested.

“He’s operated and been successful with his aspirational message of justice for everyday Americans,” he said. “Getting into the weeds … is not how he became so successful."

Several of Sanders’ closest allies on Capitol Hill expect him to stay in the race until the D.C. Democratic primary next Tuesday.

“I think so,” Rep. Keith Ellison, D-Minnesota, said in an interview, when asked if it’s possible to see Sanders stay in the race until next week. “I don’t see any problem with it.”

“He’s asked people to support him and many have, so why not give them all a chance?”

Rep. Alan Grayson, D-Florida, believes Sanders “recognizes that there are a significant number of people who want to vote for him.”

ABC's Ben Siegel contributed to this report.