Support for Democratic presidential candidate Elizabeth Warren dropped nationally to its lowest level in four months, and nearly one in three potential Democratic primary voters say they do not know which candidate to pick with the first nominating contests less than two months away, according to a Reuters/Ipsos public opinion poll.

The poll, conducted on Wednesday and Thursday, found the level of indecision has jumped among Democrats and Democratic-leaning independents as the candidate field has undergone some significant recent changes.

Sen. Kamala Harris of California quit the race this week, and two new candidates, former New York City Mayor Michael Bloomberg and former Massachusetts Gov. Deval Patrick, jumped in the race late last month.

The latest poll found that support declined for all of the top candidates, including Warren, when compared with a similar poll that ran on Nov. 20-22.

Support dropped by two percentage points for former Vice President Joe Biden to 19%. It fell by three points for U.S. Senator Bernie Sanders of Vermont to 14%, and it declined by one point to 6% for Mayor Pete Buttigieg of South Bend, Indiana.

Bloomberg, also a billionaire media mogul, entered the race as the fifth-most popular candidate with 4% support.

Support for Warren dropped by two points to 9% in the national poll, the worst showing for the lawmaker from Massachusetts in the Reuters/Ipsos poll since August.

To be sure, Warren is still among the most popular candidates in Iowa, which will be holding its nominating contest on Feb. 3, and she is also among the top candidates in other early primary states. But nationally, Warren has slipped as her rivals for the nomination have derided her proposal for extending government-paid healthcare to all Americans as too costly.

Meantime, 31% of Democrats and independents said they "don't know" which candidate to support. That is the highest level of indecision measured in the Reuters/Ipsos poll dating back to mid-April.

The poll was conducted online, in English, throughout the United States. It gathered responses from 719 adults who identify as Democrats, independents and politically unaffiliated. The poll has a credibility interval, a measure of precision, of four percentage points.