More prominent Democrats on Sunday cracked open the door for Vice President Joe Biden to enter the race for the Democratic presidential nomination as party front-runner Hillary Clinton battled questions over her use of a private email server.

California Governor Jerry Brown told NBC's Meet the Press program that Biden should give "very serious consideration" to a presidential run.

Brown said the email saga surrounding Clinton's private email server as Secretary of State carried a "dark energy" that represents a substantial challenge for her campaign.

"It is almost like a vampire. She is going to have to put a stake ... in the heart of these emails," the California Democrat said.

Asked if Biden should run, Brown said: "You are asking me presidential advice. All I can say is if I were Hillary, I would say don't jump in. If I were Joe Biden, I would probably give it very serious consideration."

Speculation grew on Saturday that Biden may soon challenge Clinton for the Democratic nomination as the vice president met with Senator Elizabeth Warren, a power broker among liberal Democrats.

Warren, who has strong support from liberal groups that would be critical to winning in early voting states, has said she will not run for president herself, but she has not endorsed Clinton or any other Democratic candidate.

Josh Alcorn, a senior adviser for Draft Biden 2016, a Super PAC group that is laying groundwork for a potential run, said the vice president was "sounding out people in early (primary election) states, activists and potential supporters."

Meeting with Warren could help give Biden more ideas for making the U.S. economy work better for middle-class Americans, he told Fox News Sunday.

If Biden decides to run, Alcorn said it would be important for him to announce his candidacy in time take part in the first Democratic debates in October.

Democratic presidential candidate Martin O'Malley said he had a "great deal of respect" for Biden.

"It would be nice to have at least one more lifelong Democrat in the race," the former Maryland governor told ABC's This Week with George Stephanopoulos.

The comment was partially a reference to independent Senator Bernie Sanders, a self-declared socialist from Vermont, who is also seeking the Democratic nomination.

Former New Mexico Governor Bill Richardson on Sunday told ABC's "This Week" program that he would endorse Clinton in her bid to win the November 2016 presidential election, but said Biden would be a formidable candidate if he entered the race.

(Additional reporting by Andrea Shalal and Sarah N. Lynch; Editing by Mary Milliken)