Vermont Sen. Bernie Sanders said Wednesday he’ll campaign actively this fall to send Democrat Hillary Clinton to the White House and bolster the party’s congressional candidates, with swings already planned in New Hampshire, New York and Washington state.

And while he has policy disagreements with Clinton, Sanders urged those who backed his defeated bid for the Democratic presidential nomination to not defect to the Green Party’s Jill Stein or Libertarian Gary Johnson.

“Politics is tough stuff, and you’ve got to make difficult choices,” Sanders, an independent, said in an interview with The Associated Press. “To me this was not a particularly difficult choice. Donald Trump would be a disaster for everything I believe in and I think my supporters believe in.”

A main motivation for his involvement would be defeating Trump, the Republican presidential nominee, Sanders said. He called Trump “the worst candidate of a major political party in my lifetime.”

“I think his election to be president would be a disaster for this country, an international embarrassment for the United States, and I’m going to do everything I can to defeat him,” Sanders said.

Speaking in his Burlington office, Sanders said it is important that Democrats regain control of the Senate.

“I think there’s a reasonable chance that that can happen,” Sanders said. “I’ll be as active as I can.”

He said he had already scheduled an appearance at an AFL-CIO breakfast in New Hampshire on Labor Day for both Clinton and Gov. Maggie Hassan, who is seeking to replace GOP Sen. Kelly Ayotte.

He said he also would campaign for House candidates, mentioning Zephyr Teachout, who is seeking a congressional seat from New York and Pramila Jayapal, who is running for Congress in Washington state.

“In our campaign there was an enormous amount of energy and grassroots activism,” Sanders said. “Obviously I do not want to see that dissipate. I want to see that level of energy continue, at all levels of government from the school board to the United States Senate.”

Sanders stood by his decision to not release a candidate personal financial disclosure this year. He got two 45-day deadline extensions from the Federal Election Commission and by late June, when it was certain Clinton would be the nominee, Sanders was excused from filing, his wife, Jane O’Meara Sanders, said in a recent interview.

“By the time this one was due I was not a candidate for president. It did not make a whole lot of sense to me to release a document that was no longer relevant,” Bernie Sanders said Wednesday.

He said he had filed a presidential disclosure last year and files the one required of senators.

– By Dave Gram, AP