WASHINGTON (Reuters) - As Joe Biden squares off against a bevy of Democratic challengers at a U.S. presidential debate on Tuesday, his fiercest opponent will not be sharing the stage with him.

FILE PHOTO: Democratic 2020 U.S. presidential candidate and former Vice President Joe Biden smiles as a supporter pumps a fist at a campaign town hall meeting in Manchester, New Hampshire, U.S., October 9, 2019. REUTERS/Mary Schwalm/File Photo

The former vice president is the only Democrat currently fighting a two-front war: one against the other contenders seeking his party’s nomination and a second, more brutal one against Republican President Donald Trump.

Some of his top backers worry Biden will lack the resources he needs for a sustained conflict. Biden has struggled with online fundraising and, unlike past Democrats, refuses donations from lobbyists and is not supported by a well-funded political action committee, known as a Super PAC.

When it comes to combating Trump, Biden is bringing “a knife to a gunfight,” said John Morgan, a Florida trial lawyer and a leading fundraiser for Biden.

Biden “doesn’t need a gun. He needs a bazooka,” Morgan added.

While the November 2020 general election remains a year away and the Democratic nominee remains to be chosen, Trump and his allies have made Biden their top target, repeatedly alleging - without offering evidence - that Biden improperly tried to aid his son Hunter’s business interests in Ukraine and China.

Trump now faces an impeachment inquiry in the U.S. House of Representatives centering on the president pressuring a vulnerable ally, Ukraine, to investigate the Bidens for his own political benefit. Trump has denied wrongdoing.

Trump’s re-election campaign has committed millions for television ads that push the same allegations against Biden, with a focus on early voting states in the party nomination battle including Iowa and New Hampshire. It has been joined in that effort by Great America PAC, a pro-Trump committee.

Biden, for his part, has tried to counter with his own early-state ad blitz that calls the president “unhinged.”

But with fundraising of $15 million in the year’s third quarter, he is facing a juggernaut in the Trump campaign, which had raised $125 million along with the Republican National Committee in the same period. Biden’s fundraising also trails his leading Democratic rivals, U.S. Senators Elizabeth Warren and Bernie Sanders, each of whom raised around $25 million last quarter.

“This is a going to be an all-pitched war fought with huge dollars,” said Joseph Falk, a top Biden fundraiser in Miami.

As distasteful as the idea has become in Democratic circles, a Super PAC may be necessary to battle Trump, some Biden donors told Reuters.

A Super PAC “would not be my preferred method,” Falk said, but “you can’t fight a battle with one hand behind your back.”

Biden will be onstage on Tuesday in Ohio with 11 other Democrats seeking the party’s nomination. The subject of Hunter Biden’s work likely will be brought up.

Hunter Biden on Sunday released a statement saying he was leaving the board of a Chinese private equity company and pledged to forego all foreign work if his father is elected president. On Monday, the Biden campaign proposed banning private money from elections and starting a new commission to enforce anti-corruption laws in a government-ethics plan that doubled as an attack on Trump.

Biden has struggled to match the grassroots appeal of Warren and Sanders, who rake in most of their funds online through small donations. Both candidates, running to the left of Biden, have made getting big money out of politics a major theme.

Biden has had to rely on traditional Democratic fundraisers, where donors write checks and mingle with the candidate.

Biden’s campaign has said there are signs that Trump’s attacks have increased donations, noting that it raised $1 million online during the first week of October alone.

It also lacks weapons employed by 2016 Democratic nominee Hillary Clinton, who accepted contributions from lobbyists and whose campaign was aided by several Super PACs.

At this point four years ago, Priorities USA, the top Clinton PAC, had raised more than $40 million to bolster her campaign’s efforts.

This time around, Priorities is not backing a single Democratic candidate, choosing instead to run ads attacking Trump in battleground states.

“Our singular focus is on defeating Trump, and it’s up to Democratic voters” to pick their nominee, said Josh Schwerin, the group’s spokesman.

‘LIES AND SLANDER’

The Biden campaign believes Super PACs, which can receive unlimited donations, are a corrupting influence in politics.

“The attacks aimed at this campaign from dark-money groups helping Donald Trump spread his outlandish lies and slander have only served as a reminder of the urgent need for campaign finance reform,” said TJ Ducklo, a Biden spokesman.

Dark money refers to funds from outside political groups, with the identity of the donors undisclosed.

Rufus Gifford, who oversaw Barack Obama’s 2012 presidential re-election fundraising efforts, said the Democratic Party needs “to do everything we can with the stakes as high as they are.”

“If there wasn’t a threat to our democracy, we can have this talk about the moral high ground,” Gifford said.

Other Biden backers criticize the Democratic National Committee (DNC) and its chairman, Tom Perez, for not doing enough to defend Biden against what they see as Trump’s dishonest attacks.

“There’s no pushback on Trump, no message,” said Dick Harpootlian, a Biden fundraiser in South Carolina and former state party chair.

Harold Schaitberger, general president of the International Association of Fire Fighters, a union that has endorsed Biden, expressed frustration over “a lot of silence” from the DNC and other party leaders.

“It’s crickets,” Schaitberger said.

After being criticized in 2016 for appearing to favor Clinton, the DNC has sworn to remain neutral this time while still attacking Trump. “It’s not their job to be an ad-making operation for Joe Biden,” said one Democrat with knowledge of the DNC’s activities.

Biden’s donors remain optimistic about his prospects and came away encouraged from an early October meeting of about 100 donors in Philadelphia. Biden fundraiser Falk said attendance was better than expected and enthusiasm was high.

A front-runner for much of the 2020 Democratic race, Biden has not lost significant support in national polls since Trump’s request to Ukraine came to light.

Ultimately, though, a successful 2020 Democratic nominee will have to be able raise money online and from traditional high-dollar fundraisers to defeat Trump, Falk said.

“You can’t go into a race,” Falk said, “being outspent two or three-to-one and win.”