"From the beginning of this campaign Sen. Sanders has called for more debates. Secretary Clinton has not. Now she is asking to change the rules to schedule a debate next week that is not sanctioned by the DNC. Why is that? The answer is obvious. The dynamics of the race have changed and Sen. Sanders has significant momentum," said Sanders campaign manager Jeff Weaver in a statement on Wednesday night, while Sanders was speaking to a packed audience here.

"Sen. Sanders is happy to have more debates but we are not going to schedule them on an ad hoc basis at the whim of the Clinton campaign. If Secretary Clinton wants more debates that's great. We propose three additional debates. One in March, April and May and none on a Friday, Saturday or holiday weekend. And all of the three Democratic candidates must be invited. If the Clinton campaign will commit to this schedule, we would ask the DNC to arrange a debate in New Hampshire on Feb. 4."

The move puts the burden of response back on the Clinton camp, which had publicly urged Sanders to join the proposed New Hampshire event, which would be co-hosted by the New Hampshire Union-Leader and MSNBC. But the DNC has refused to sanction that debate, and has shown no willingness to accept any further events.

Clinton has said she would only debate if the DNC agreed to approve the event, as the DNC has said candidates who appear in any unofficial events would be barred from the previously scheduled ones. The next official debate is set for Feb. 11 in Milwaukee.

The long-simmering debate over the debates boiled over on Tuesday after MSNBC and the Union-Leader announced their event. Sanders and Martin O'Malley have long contended that there are not enough debates — there were just four official ones before the Iowa caucus — and that they were deliberately scheduled at odd times to lower viewership and protect Clinton.

But Clinton has said she would participate in new debates if the DNC agrees to them, and she has recently signaled a willingness to push for more.

"What I’ve said to my campaign is that I would look forward to another debate," she told MSNBC's Chris Matthews on Wednesday. "I am, you know, anxious, if we can get something set up, to be able to be there. And so let’s try to make it happen."

The Clinton campaign did not immediately respond to a request for comment on the Sanders campaign's challenge.