Former Gov. Deval Patrick of Massachusetts told senior Democrats Wednesday that he will enter the presidential race, according to two party officials, reversing his decision last year to forgo a run and adding yet more volatility to an unusually fluid Democratic primary.

Mr. Patrick was calling a list of Democrats to inform them of his decision and is expected to begin his campaign with a video before appearing in person in New Hampshire to file his paperwork to be on the primary ballot there. He did not immediately reply to a text message.

Joining the campaign less than three months before the first votes are cast in Iowa, at a moment when candidates are usually dropping out and not jumping in, Mr. Patrick will face long odds. Yet his decision to run reflects the fractured nature of the Democratic race at a moment when Michael R. Bloomberg, the former mayor of New York City, has also taken steps to enter the primary.