Democratic presidential candidates approach their first televised debates in a scramble for the 2020 nomination, with former Vice President Joe Biden a vulnerable front-runner, a new NBC News/Wall Street Journal poll shows.

In the survey, 66% of Democratic primary voters say they'd be enthusiastic or comfortable about Biden as their nominee to take on President Trump in the 2020 election. Just 32% express reservations.

Both assessments reflect modest erosion for Biden since March, when 73% were enthusiastic or comfortable and 25% expressed reservations. Debates in Miami on June 26-27, sponsored by NBC, MSNBC and Telemundo, will give 19 rivals a chance to further soften Biden's support.

The former vice president's standing is "very solid," said Bill McInturff, the Republican pollster who conducts the NBC/WSJ survey with Democratic counterpart Peter Hart. But he added, "Biden's numbers don't look like those of a prohibitive front-runner."

The survey reflecting a similar softening of support for Sen. Bernie Sanders, the self-described Democratic socialist from Vermont. Fully 43% of Democratic primary voters express reservations about him, up from 36% in March, while 56% are enthusiastic or comfortable, down from 62% in March.

One Democrat making significant progress during that time is Sen. Elizabeth Warren of Massachusetts, who has earned positive reviews for offering detailed plans designed to improve economic opportunities for average families. Now, 64% say they'd be enthusiastic or comfortable about her, up from 57% in March, compared to 27% with reservations, down from 33% in March.

At the same time, many in the crowded Democratic field remain unknown to national audiences. For example, more than seven in 10 Democrats say they don't known enough to assess Gov. Jay Inslee of Washington or Sen. Michael Bennet of Colorado. That gives both men a chance to make first impressions on the debate stage.

Current assessments of the candidates "are going to change," pollster Hart cautioned. "They're fluid."

One factor with the potential to influence the Democratic debate is rising support for impeachment proceedings against President Trump. Though House Speaker Nancy Pelosi has held off so far, the share of Democrats who want to move toward impeachment has grown to 48% from 30% in April after release of special counsel Robert Mueller's report.

Support for impeachment proceedings has also ticked up among independents, from 19% to 22%, and Republicans, from 3% to 6%. Overall, 51% say Congress should either begin impeachment hearings now or keep investigating and decide later, while 48% say Trump should be allowed to complete his term.

At the same time, approval of Trump's performance as president has ticked down to 44% of Americans from 46% in late April. A 53% majority disapproves, up from 51%.

In an ominous sign for his re-election prospects, fully 62% express reservations about Trump winning a second term. Just 23% describe themselves as enthusiastic about the president in 2020, while 52% call themselves "very uncomfortable" with his candidacy.

The telephone survey of 1,000 adults was conducted June 8-11, It carries a margin for error of 3.1% points.