ABC News' Ed O'Keefe reports Sens. Hillary Clinton, and Barack Obama, "could debate at least twice more before the critical Pennsylvania primary on Tuesday, April 22."

Both candidates have accepted an ABC News debate in Philadelphia and Obama has accepted an invitation from CBS News to debate in North Carolina. CBS had planned for a Democratic debate in December in California, but the DNC canceled the event, "Due to the uncertainty created by the ongoing labor dispute between CBS and the Writers Guild of America."

ABC News hosted back-to-back GOP and Democratic debates in January moderated by Charles Gibson.