Bernie Sanders addresses the Democratic National Convention in Philadelphia on Monday. (Toni L. Sandys/The Washington Post)

PHILADELPHIA — Sen. Bernie Sanders rebuffed calls from the Green Party — and a loud minority of his delegates and supporters here — to abandon the Democratic Party and build a third force outside it.

"I don't know the leadership of the Green Party, but I respect what they're trying to do," Sanders said, at a breakfast sponsored by Bloomberg Politics. "They're focusing on very, very important issues. But I think right now — what is it, three, four months before an election — you're going to end up having a choice. Either Hillary Clinton is going to become president, or Donald Trump."

The Green Party, which has never again reached the heights of its 2000 performance under Ralph Nader, has held a series of Philadelphia events since the weekend aimed at persuading Sanders voters to make a "DemExit." Jill Stein, the party's likely nominee, has spoken even as rainstorms shut down meetings, repeatedly calling on Sanders to quit the Democrats and talk to her; Cornel West, a Sanders supporter who was one of his five appointees to the party's platform drafting committee, has appeared at Stein events and canceled at least one Democratic event around the convention.

Sanders, meanwhile, has declined to meet with the Greens and urged his supporters to stay and change the Democratic Party.

"We are not a parliamentary system, as is the case in Europe," Sanders said. "If we were in Europe right now, in Germany or elsewhere, the idea of coalition politics of different parties coming together — you've got a left party, you've got a center-left party, coming together against the center-right party. That's not unusual. That happens every day. We don't have that. We have and have had [two parties] for a very long period of time — and I know a little bit about this, as the longest serving independent member of Congress."

At the same time, Sanders suggested that the thousands of people who signed up to run for office, on his suggestion, would not work inside Hillary Clinton's party.

"I expect that most of them would run as Democrats," Sanders said. "Some may run as independents."

Asked whether all of his voters would back the Clinton ticket, Sanders said it was not possible — or even desirable — to force them to.

"We live in a democratic society," Sanders said. "The role I will play is being very boring and contrasting what Donald Trump wants for America with what we want."

Sanders was also unable to rule out that some of his supporters would end up voting for the Republican nominee, who has fitfully attempted to court them by saying they were disrespected by the Democrats and that only he would kill the Trans-Pacific Partnership.

"Thirteen-point-three million people," said Sanders, referring to his total primary vote. "Do I think some of them will vote for Donald Trump? What can I tell you."