Bernie Sanders has generated a lot of excitement at his rallies and at Democratic primaries this spring. But he's generated decidedly less excitement among his Senate colleagues; just one fellow senator has endorsed him — Sen. Jeff Merkley of Oregon.

Most of Merkley's colleagues have endorsed Hillary Clinton, the Democratic front-runner who has a lead of a few-hundred pledged delegates, and a dominant lead among superdelegates. But even as Merkley cuts a lonely figure riding the Sanders train, he says some of his fellow senators regret endorsing Clinton so early in the nomination process — and that if Democrats want to win in November, they'll have to bring Sanders supporters into the party fold in a way they just haven't seemed to be able to stomach so far.

In an interview with The Fix on Tuesday, Merkley said the party has to make very careful calculations about getting some of Sanders's campaign positions onto the party platform, saying Democrats "cannot slam the door on folks, and then reopen it up and say, 'By the way, come and help us out.' "

"For them to come on to the team. ... You need that energy," he said. "And you don't do that by saying to folks, 'Hey, we're not interested in your opinion. We don't want you to be able to vote. We want to pull the rug out from under Bernie Sanders's campaign.' No. You honor and respect them, and say, 'Yes, let your voice be heard.' "

Merkley had strong words for the Democratic National Committee (and by association, DNC Chairman Debbie Wasserman Schultz), against whom The Fix said earlier this week that Sanders has "declared war."

"The DNC has not been a fair arbiter of planning the convention," he said. "Unfortunately the DNC — until [Monday]— was not doing its part. It was pouring gasoline on the fire. It was deepening the divide between the campaigns. I can't tell you how frustrating that is to so many Democrats who know that we have to come together."

But progress is being made, Merkley said.

"[Monday] was really the pivot point," he said. "I'm thrilled about the fact that a deal was worked out for the platform committee [for the Democratic National Convention in July]. ... That's the type of respectful work that will pave the path for people coming together."