Presumptive Democratic presidential candidate Hillary Clinton and Bernie Sanders wave Tuesday after speaking at a rally in Portsmouth, New Hampshire. Getty Images

Bernie Sanders may have endorsed rival Hillary Clinton for president, but he's not ready to give up quite yet. Looking for proof? Sanders, the Vermont senator who proved particularly popular among young voters in the Democratic primary, has not given Clinton's campaign his highly desirable email list, an NBC producer tweeted Wednesday afternoon.

Sanders raised more than $222 million over the course of his campaign, with thousands of donations coming from grassroots volunteers and small-time supporters. Their contact information is contained in his email list, which Politico recently called "perhaps the most coveted and valuable catalog of potential voters and donors in the Democratic Party at the moment." Clinton almost certainly wants it.

But he hasn't shared the data yet, and some speculate that he never will. "Young people have been very clear that they don’t want their information handed over and it’s not going to be," RoseAnn DeMoro, the executive director of the Sanders-supporting group National Nurses United, told BuzzFeed last month. "Sanders could surprise me, but I don’t think so.”

The senator has not formally ended his campaign, even though he admitted at a news conference Tuesday that "Secretary Clinton has won the Democratic nominating process" and "she must become our next president."

A Sanders spokesman confirmed to the Atlantic that the candidate was not dropping out. Sanders will have roughly 1,900 delegates at the Democratic National Convention July 25-28 in Philadelphia.

He promised Tuesday to continue his political revolution, feeding rumors that he may use his email list to support progressive candidates further down the ballot, Politico reported.

“Our goal will be to advance the progressive agenda that we believe in and to elect like-minded candidates at the federal, state and local levels who are committed to accomplishing our goals,” USA Today reported Sanders wrote in a recent email to supporters.