MASON CITY, Iowa — With just under two months until the Iowa caucuses, the already-volatile Democratic presidential race has grown even more unsettled, setting the stage for a marathon nominating contest between the party’s moderate and liberal factions.

Pete Buttigieg’s surge, Bernie Sanders’s revival, Elizabeth Warren’s struggles and the exit of Kamala Harris have upended the primary and, along with Joseph R. Biden’s Jr. enduring strength with nonwhite voters, increased the possibility of a split decision after the early nominating states.

That’s when Michael R. Bloomberg aims to burst into the contest — after saturating the airwaves of the Super Tuesday states with tens of millions of dollars of television ads.

With no true front-runner and three other candidates besides Mr. Bloomberg armed with war chests of over $20 million, Democrats are confronting the prospect of a drawn-out primary reminiscent of the epic Clinton-Obama contest in 2008.