DAVENPORT, Iowa—The unity between the Democratic primary’s two liberal firebrands, Sens. Bernie Sanders and Elizabeth Warren, is being tested as Iowa’s caucuses next month loom over the 2020 race.
Ms. Warren, who has lost ground in polling and fundraising in recent months, seems loath to directly needle the Vermont lawmaker on just about anything. But some of Ms. Warren’s allies say Mr. Sanders’s identity as a self-described Democratic socialist could be a vulnerability for him—and, by extension, for her, if more moderate…