Bernie SandersBernie SandersWarren to protest with striking Chicago teachers Sanders: 'Outrageous' to suggest Gabbard 'is a foreign asset' Democratic strategist: Sanders seeking distance from Warren could 'backfire' MORE on Wednesday called Hillary ClintonHillary Diane Rodham ClintonSanders: 'Outrageous' to suggest Gabbard 'is a foreign asset' Clinton attacks on Gabbard become flashpoint in presidential race Saagar Enjeti: Clinton remarks on Gabbard 'shows just how deep the rot in our system goes' MORE's higher education proposal "a major step forward," but he wouldn't give a timeline for his endorsement of the presumptive Democratic presidential nominee. 

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The Clinton campaign earlier in the day announced a proposal to eliminate college tuition for families making less than $125,000 a year. 

"That is a revolutionary breakthrough for the middle class and working class of this country," Sanders said on CNN Wednesday. 

Sanders, who has said he will remain in the presidential race until the Democratic National Convention later this month, has lobbied Clinton for changes to her policy positions and to the party's platform. He has frequently called her to go further on issues like higher education and trade, and said Wednesday the two are still working toward an endorsement. 

When asked if he is fighting for a Democratic nominee to win, Sanders said, "You bet your bottom dollar on that." 

Sanders has also pushed for concessions in the Democratic platform, and a draft approved recently includes his calls for a $15 minimum wage, an expansion of Social Security and an end to the death penalty.

But the Vermont senator said there is still work to be done. 

"What I am trying to do, and the reason I ran for president, is to help transform this country," Sanders said. 

"What I want to do is make sure we have the strongest Democratic platform out there that represents working families, and we have made good progress on that."