Joe Biden is rising ...

Vice President Joe Biden isn't even a presidential candidate yet, but he's surging in a new national poll of the Democratic primary race.

A new Monmouth University poll of national Democratic voters shows Biden - who is weighing a run - gaining on Democratic presidential front-runner Hillary Clinton. He's also slightly ahead of Sen. Bernie Sanders (I-Vermont) for the first time among likely Democratic voters.

According to Monmouth, Biden grabs 22% of the vote nationally among Democratic voters, compared to Sanders' 20%.

Both trail Clinton, who has 42% support, but both have steadily gained steam over the last several months.

"For a guy who is not running for president, Biden sure is making headway against the frontrunner," said Patrick Murray, the director of the Monmouth University Polling Institute.

"There also seems to be an opening with the more liberal Sanders voters if Biden plays his cards right."

Biden is still weighing a run, saying last week that he is considering it but that he's not sure he has the emotional energy for another bid at the presidency following the death of his son, Beau, in May. He heard chants of, "Run, Joe, run!" at a Labor Day parade in Pittsburgh on Monday, but he didn't give supporters any hints of his plans.

Since announcing his candidacy earlier this year, meanwhile, Sanders has rocketed up polls, overtaking Clinton in several surveys of New Hampshire, the first-in-the-nation primary state.

Biden has been watching Sanders' rise closely, and told donors earlier this month that he was impressed by Sanders' ability to excite voters.

"I am not a populist. But Bernie Sanders, he's doing a hell of a job," Biden said, according to Politico. He made similar remarks Monday while participating in the Labor Day parade, while avoiding mention of Clinton.

Despite Biden's rising levels of support among Democrats, he would still have huge obstacles to overcome if he chooses to jump into the race.

Beyond the obvious problems of launching a campaign so late into the race, including a lack of a campaign organization and a huge fundraising deficit compared to his rivals, Biden is also not viewed much more favorably among the Democratic base.

According to Tuesday's Monmouth poll, 71% of Democratic voters view both Biden and Clinton in a positive light, though more Democrats view Clinton unfavorably.

Biden, though, has a higher favorability rating among Republicans and Independents. A Washington Post/ABC poll released earlier this month showed that 26% of Republicans have a positive view of Biden, compared to 13% who have a positive view of Clinton.