"We have got to do everything that we can to defeat Donald Trump and elect Hillary Clinton," Sanders told Bloomberg's Al Hunt. "I don't honestly know how we would survive four years of a Donald Trump" administration, he added.
The remarks will be aired on the Charlie Rose program.
Though Sanders has yet to formally drop his bid for the Democratic nomination, the Vermont senator has made clear that he'll back Clinton, the presumptive nominee, in her general election face-off against Trump.
Sanders told MSNBC on Thursday he was "working with the Clinton campaign" to vanquish the GOP standard-bearer, one day after he praised Clinton for moving closer to his call for free public college with her proposal for tuition-free higher education for student from families earning less than $125,000 annually.
In late June, Sanders said he would vote for Clinton in the general election, though he seemed to walk back those remarks when he later said he'd only be voting for her "in all likelihood."
Sanders plans to carry his campaign through the Democratic National Convention in Philadelphia later this month, where he hopes to move the party platform leftward.
But he may begin campaigning with Clinton as soon as next week. The Clinton campaign announced Thursday she would be campaigning Tuesday in New Hampshire — where Sanders handily defeated her in the February primary — and while no announcement has yet been made, the Clinton and Sanders camps have been in talks about a joint event in the state.
4:05 p.m. Eastern: This story has been updated.