The anxiety is the result of an unusually wide-open race. Less than three months before Democrats cast their first primary votes, none of the party’s presidential candidates is polling regularly above 30 percent. At least four are on track to earn delegates in Iowa. And now, former New York mayor Mike Bloomberg, one of the world’s richest people, says he might spend hundreds of millions of dollars to attract voters down the stretch.

Broadly, the Democrats’ fear is that the surging energy and turmoil in their party, and its passion to defeat President Trump, may be leading to fracturing rather than unity. Some in the party worry that could hamper its ability to defeat Trump, although others argue that a tough primary will strengthen the eventual nominee…

The operatives point to other factors, including the increasing role of small-dollar fundraising, which allows candidates to stay in longer, and the diminished role of superdelegates, whose function was to rally the party around a nominee if needed.

They also point to Sen. Bernie Sanders’s medical and political rebound from a heart attack, which suggests he could battle Sen. Elizabeth Warren for liberal voters for months. One delegate counter admitted to concluding it is now “more likely than not” that no one gets a majority of pledged delegates, largely because of Sanders’s continued strength and the likelihood that he will continue to campaign even if he is trailing late in the primary contest.