Continued from page 1

Progressive darling Sen. Elizabeth Warren, Massachusetts Democrat, is widely viewed as one of the few who could mount a serious challenge to the Clinton juggernaut, but she’s said repeatedly she won’t seek the White House in 2016.

Other prospective Democratic candidates, such as Vice President Joseph R. Biden and Maryland Gov. Martin O’Malley, trail far behind Mrs. Clinton in recent surveys.

But even if she avoids a viable threat from the left, Mrs. Clinton still may be taking a serious risk by alienating some in the party’s hard-core liberal base.

“There are a lot of progressives who like her and want her to win but don’t especially want to see her attacking the president,” Mr. Hickey said.