Former Vice President Joe Biden leads President Donald Trump by nine points nationally, and his advantage in key battleground counties is even greater, according to a new Fox News poll.

Biden leads Trump 49-40, according to the survey, which is roughly the same margin he held a month ago in the right-leaning network's last poll. The poll shows that Biden leads the president by eight points in key battleground states and by 25 points, 57-32, in key battleground counties decided by fewer than 10 points in 2016.

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Trump has repeatedly groused about Fox News polling, which has consistently shown Biden leading by as many as 11 points nationally. After last month's poll showed him losing by nine points, the president complained that the network had the "worst polls." However, election forecaster FiveThirtyEight grades its pollsters quite highly.

"Whoever their Pollster is, they suck," Trump tweeted.

Other recent data is far less bullish on Biden's chances against Trump. The race has been upended by the coronavirus pandemic, which has forced Biden off the campaign trail while networks air Trump's daily briefings live. The president's approval ratings have also ticked up during the crisis, though they remain net negative.

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A Monmouth poll published last week showed Biden leading by just three points, 48-45, though he leads by nine points in the 300 swing districts decided by fewer than 10 points.

A Washington Post/ABC News poll published last week showed Biden up 49-47 in a survey with a margin of error of 3.5%. The outlets' last poll, conducted in February, showed Biden leading by seven points.

Biden, who trounced Sen. Bernie Sanders, I-Vt., in the March primaries, has championed a message of "electability."

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The poll finds that 86% of Trump supporters are "enthusiastic" about voting for him in November in comparison to 74% of voters who say they plan to vote for Biden. But 55% of Trump supporters say they are "very enthusiastic" about backing the president, compared to just 28% of Biden's supporters.

Such wide enthusiasm gaps have spelled trouble for campaigns. Mitt Romney had a 25-point strong enthusiasm deficit in 2012, former Sen. John McCain, R-Ariz., had a 33% enthusiasm deficit in 2008 and former Sen. John Kerry, D-Mass., had a 16-point enthusiasm deficit in 2004, The Post noted. All three went on to lose their elections.

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A key factor for Biden will also be his ability to reach out to Sanders' supporters. The poll finds that 79% of current Sanders backers plan to vote for Biden against Trump, compared to 84% of Sanders voters who said they would back Hillary Clinton in November 2016. But Biden is actually outperforming Clinton, who had just 71% support from Sanders' backers at this point in the race in 2016.

The state of the Democratic primary race, where Biden holds a nearly insurmountable lead, remains in flux. Many states have postponed their primaries amid coronavirus fears, and it is unclear if there will be any further debates. Sanders said last week that he was prepared to debate Biden next month, but Biden was uninterested.

"I think we've had enough debates," Biden said. "I think we should get on with this."

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With the campaign disrupted by the coronavirus, it is unclear how Biden can reach key voting groups.

"The enthusiasm gap recorded in the poll is troubling," Democratic strategist Brad Bannon told The Hill. "Everything the president has done since his inauguration was designed to fire up the Republican base, and the effort has paid off."

"It should be worrisome," agreed Julian Zelizer, a professor of history and public affairs at Princeton University. "Given how President Trump has performed in this crisis, support for Biden should be through the roof, but it's not."

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But Democratic strategist Joel Payne argued that record turnout in some states that Biden won show that the enthusiasm gap is overblown.

"The best indicators for Biden are the strong Democratic turnout stats in a number of the primary states through early March," he said. "The turnout numbers we've been seeing mirror Obama-level turnout. What's more impressive for Biden is that the spike in turnout came from older voters and voters in the suburbs. That's the way Democrats can win, and that's the wave that Biden can ride to the White House."