A new national opinion poll conducted by Monmouth University shows Sen. Elizabeth Warren, D-Mass., and former Vice President Joe Biden as the leading Democratic presidential candidates, though the poll also shows support for President Donald Trump's reelection to be unchanged since House Democrats opened an impeachment inquiry.

Among registered Democratic or Democratic-leaning voters, 28% preferred Warren, and 25% preferred Biden as the Democratic nominee.

Sen. Bernie Sanders, I-Vt., came in third place at 15%.

Shortly before the poll's release, Sanders was hospitalized after suffering chest discomfort at a campaign event in Las Vegas.

"We are canceling his events and appearances until further notice, and we will continue to provide appropriate updates,” said campaign adviser Jeff Weaver.

The poll shows support for President Donald Trump's reelection among registered voters at 39%, an identical finding to Monmouth's August poll. His approval rating was 43%, and his unfavorable rating was 56%.

Both South Bend, Ind., Mayor Pete Buttigieg and Sen. Kamala Harris, D-Calif., polled at 5%.

Author and activist Marianne Williamson and entrepreneur Andrew Yang polled at 2%.

All other candidates polled at 1% or less.

This poll, one of the qualifying polls for the fifth Democratic presidential debate in November, does not change the makeup of the debate stage. Sen. Cory Booker, D-N.J., and businessman Tom Steyer polled at 1%, and Yang's polled at 2%, all below the 3% debate qualification threshold.

More:2020 White House hopeful Bernie Sanders hospitalized after suffering chest pains

More:Poll: Only 4 in 10 Republicans think Trump mentioned Biden on Ukraine call even though he acknowledged doing so

Monmouth also surveyed voters about the controversy surrounding Trump's alleged pressuring of the Ukranian government to open an investigation into Biden.

According to the poll, 43% of registered voters believe Trump's allegation that Biden had interfered in a Ukranian government investigation into his son Hunter Biden's business dealings, though belief in that claim breaks along partisan lines.

67% of Republicans believed Trump's claim, in comparison with 19% of Democrats, and 45% of independents.

Monmouth University Polling Institute director Patrick Murra argued this finding could spell trouble for the Biden campaign

"The fact that 4-in-10 independents are inclined to believe what they have heard from Trump is a warning sign for the Biden campaign. How the candidate fights back against this charge will be crucial to his argument of electability," said Murray.

The poll of 434 registered Democratic voters was conducted from Sept. 23-29, with a 4.7 percentage point sampling margin of error.

For the results about Trump's approval and the Ukraine controversy, Monmouth surveyed 1,018 registered voters, with a 3.1 percentage point sampling of error.