But perhaps a more serious liability for her campaign will be her interactions with Syria’s president, Bashar al-Assad. A longtime opponent of the United States’ military involvement in Syria, Ms. Gabbard visited Mr. Assad in Damascus in January 2017 — something other American officials had not done since Mr. Assad’s use of chemical weapons against civilians became widely known. Her noninterventionist perspective and her desire for diplomacy could appeal to many progressive Democrats, but her decision to meet with a man who had attacked his own people angered many others.

“I think we should be ready to meet with anyone if there’s a chance it can help bring about an end to this war, which is causing the Syrian people so much suffering,” she said at the time.

Republicans quickly jumped on her campaign announcement, emphasizing her visit with Mr. Assad and declaring that Ms. Gabbard had no political base.

“Liberals think she’s too conservative, conservatives think she’s too liberal, and just about everyone thinks her coziness with Bashar al-Assad is disturbing,” said Michael Ahrens, a spokesman for the Republican National Committee.

Ms. Gabbard is one of the first Democrats to declare her candidacy — Senator Elizabeth Warren jumped in on New Year’s Eve, the former West Virginia state senator Richard Ojeda did so in November, and former Representative John Delaney has been running for a year and a half — but the floodgates are likely to open in the coming weeks.

Announcements are possible from Ms. Harris, Mr. Sanders and Senators Cory Booker, Kirsten Gillibrand and Amy Klobuchar, as well as former Vice President Joseph R. Biden Jr. and Beto O’Rourke, the former Texas congressman who gained enormous attention for his unsuccessful campaign against Senator Ted Cruz last year.