A clear top tier is emerging in the 2020 Democratic race — but the likely eventual nominee is far from certain, an ABC News/Washington Post poll published early Sunday shows.

Why it matters: Per the Washington Post, the race is "competitive and fluid less than 100 days before the Iowa caucuses, with a stable trio of leading candidates" — the frontrunner, former Vice President Joe Biden (27%); Sen. Elizabeth Warren (21% ); Sen. Bernie Sanders (19%); and fourth-placed South Bend Mayor Pete Buttigieg (9%) breaking away from other single-digit rivals.

About half of Democrats polled who support a candidate say they’d consider another one.

Worth noting: Sanders’ health is a concern for a large portion of the of 1,003 Democrats surveyed in the poll several weeks after he had a heart attack. He's vowed to run a "vigorous campaign.

"The poll notes that "younger adults, who are among Sanders’ strongest supporters, are most apt to think he’s in good enough health to serve — six in 10 18- to 39-year-olds say so. Just 40 percent of those age 40 and older agree."

Read the full poll:

Go deeper: The problem with 2020 presidential polls