COUNCIL BLUFFS, Iowa — Jackie Wellman worries that Senators Bernie Sanders and Elizabeth Warren are too liberal to defeat President Trump, thinks Mayor Pete Buttigieg of South Bend, Ind., may be too inexperienced and, while she is fond of Joseph R. Biden Jr., she is also uneasy about his tendency to misspeak. “I am worried about the top four candidates because they all have issues,” said Ms. Wellman, a 54-year-old Iowa caucusgoer from West Des Moines who likes Senator Amy Klobuchar.

Ms. Wellman is not the only Democrat who keeps finding flaws as she searches for the best candidate to win the White House.

“Voters are holding back because just when they start to fall in love, they find something that gets them a little nervous,” former Mayor Rahm Emanuel of Chicago said. “The reason it’s so fluid is because they’re still searching for the horse that can win.”



Michael R. Bloomberg believes that may be him. But Mr. Bloomberg’s apparent decision to mount a late entry into the presidential race represents something more than just the robust self-confidence of a New York City billionaire; it’s a manifestation of the Democratic angst that has been growing, particularly among moderate voters and party establishment figures.

Mr. Bloomberg has jolted the Democratic primary, drawing fire from the leading liberals in the field who said he was trying to buy the presidency while posing a direct threat to the centrist candidacy of Mr. Biden. But he has also exposed the jitters among establishment-aligned Democrats who fear that the leftward turn of the party is endangering their chances of building a winning coalition .



For weeks, senior Democratic officials and donors have been musing about whether a new candidate could be lured in the race, a striking illustration of nervousness just three months before the Iowa caucuses. Some talked up Mr. Bloomberg and Hillary Clinton, but others wondered if Senator Sherrod Brown of Ohio might make a late entry to unite a party splintered along ideological lines.