Even before Graham Platner withdrew from Maine’s Senate race in disgrace this month, he and his Democratic allies were severely outmatched in the race for cash.

Now his expected replacement, Troy Jackson, inherits a difficult situation: He has just over four months to erect a defense against a towering Republican super PAC apparatus that is intent on re-electing Senator Susan Collins.

Ms. Collins is relying on a web of deep-pocketed outside groups, funded largely by billionaire megadonors, to spend tens of millions of dollars on advertising promoting her and attacking the Democratic nominee. Her campaign, her allies and other groups aligned with Senate Republican leadership have already reserved about $60 million in airtime through the fall, according to the media tracking firm AdImpact.

That is far more than the roughly $24 million that Senate Democrats’ main super PAC and its affiliates have allocated to ads in Maine, though more could be on the way.

With Republicans set to outspend Democrats that drastically, a bruising, asymmetrical battle is about to unfold for a seat that could be pivotal to control of the Senate.

“We’re probably never going to be able to match what the Republicans are going to spend, but we’ve got to have a fight,” said Rufus Gifford, a leading Democratic fund-raiser who hosted an event for donors to Mr. Platner in May. He added, “This is not a level playing field by any stretch of the imagination.”

Mr. Jackson, a former state lawmaker and logger from rural Northern Maine, is a progressive who has railed against billionaires, has few ties to Democratic big donors and has never been a strong fund-raiser.

He is expected to become the Democratic nominee at a convention on Saturday. Once that happens, he will have to build an expansive national fund-raising operation and donor list from the ground up even as he works to court voters across Maine.

“I’m going to definitely bust my ass in these next four months to make up the difference” in money, Mr. Jackson said in an interview on Monday with The New York Times. “And in the end, I think I’m on the right side of the issues compared to Susan Collins. So I feel very good about the race.”

He estimated he would need between $40 million and $50 million, but added, “That’s probably not even enough.”

The Collins Fund-Raising Machine

Over nearly three decades as a senator, Ms. Collins has built a deep bench of supporters across the country. And she has had six years to prepare for a tough re-election battle.

She is slated to benefit from the support of outside groups more than any other Republican candidate this cycle, according to advertising data. Including expenditures from during the primary, the Republican ad campaign for her is currently set to cost $113 million in total.

Her allies have set up a deep-pocketed super PAC, Pine Tree Results, that is independent from those controlled by national Republican leaders.

The group poured millions into a series of tough attack ads against Mr. Platner and is planning to spend about $20 million more through Election Day on television and digital ads.

“We plan to welcome Jackson to the N.F.L. quickly,” said Scott Reed, who runs Pine Tree.

Amid the onslaught, Mr. Platner had criticized Pine Tree for relying on out-of-state donors. Of the $16.5 million in contributions the group has received, $5,000 — or 0.03 percent — came from within Maine, according to an analysis of campaign finance reports.

Ms. Collins’s campaign said that the donations from outside Maine were a testament to her experience and her bipartisan record.

“She is receiving support from across the country because, in these tumultuous, highly polarized times, people recognize Senator Collins’s steady, reliable presence in the Senate,” Blake Kernen, a campaign spokesman, said in a statement.

Pine Tree has received seven-figure checks from the investor Paul Singer, the hedge fund titan Ken Griffin, the private equity chief Stephen A. Schwarzman and Jim Davis, the owner of the footwear company New Balance. None of whom live in Maine, which is known as the Pine Tree State. Mr. Singer is also backing another pro-Collins super PAC called Winning for Women.

Pine Tree has raised $20 million this cycle and had $8 million left in the bank as of June 30, according to the most recent filings, while Winning for Women had nearly $700,000 on hand at that time. Ms. Collins has $12 million in her own campaign accounts, including her leadership fund, Dirigo PAC.

Overall, Ms. Collins and her allied groups collected donations from at least 79 billionaires since January 2025, according to an analysis in June by The Maine Monitor.

Senate Republicans’ main super PAC is also backing her with tens of millions of dollars.

At a time of public anger at inequality, Mr. Jackson may be able to seize on Ms. Collins’s support from billionaires. But allies of Ms. Collins say they have seen such attacks fail before.

“People know her — they know she doesn’t live a lavish lifestyle,” said Lance Dutson, a Republican political strategist who has worked for the senator on her previous campaigns. “There’s just none of the trappings of what someone would see as an associate of the billionaire class.”

A Blue-Collar Candidate in a High-Dollar Race

In some ways, Democrats’ candidate switch-up in Maine could help level the playing field.

Democrats have already lobbed millions of dollars’ worth of ads against Ms. Collins. But her operation will now have to devise messaging about a new opponent, after spending months planning how to hammer Mr. Platner over his many controversies.

Those scandals had made some Democratic big-dollar donors hesitant to support his campaign. Mr. Jackson can now try to bring them back into the fold.

But in Mr. Jackson’s previous campaigns, he struggled to raise money. He lagged behind competitors in reported contributions for his unsuccessful primary campaigns for Congress in 2014 (he raised around $300,000) and the governor’s mansion in June (he raised $1.2 million).

After Mr. Jackson’s backers swept delegate elections last weekend, his campaign announced that it had raised $1 million in less than two weeks — a sizable number given the short timeline, but one unlikely to spook Ms. Collins.

Mr. Jackson is likely to have other help, however.

After Mr. Platner withdrew, Senate Democrats’ campaign arm and the Maine Democratic Party established a fund to be transferred to whoever emerges as the new nominee. Mr. Jackson is also likely to lean on groups aligned with organized labor, similar to a state PAC that spent nearly $1 million in ads emphasizing his blue-collar bona fides in the governor’s race.

And though Mr. Jackson told The Times on Monday that he would not support Senator Chuck Schumer, the minority leader, the super PAC aligned with Mr. Schumer has kept its millions of dollars in ad reservations in Maine through November.

“Leader Schumer has one message for every Democratic Senate candidate: Just win,” said Allison Biasotti, Mr. Schumer’s spokeswoman.

There will also be no shortage of small-dollar donors, who tend to flock to major Democratic campaigns for Senate, especially at the end. Nearly 25,000 donors have pitched in for Mr. Jackson’s Senate bid so far, according to his campaign.

‘National Donors Are Going to Have to Step Up’

Money alone does not guarantee victory for either party.

In a state like Maine, which has roughly one million registered voters and just three media markets, strategists say that huge political spending can reach a saturation point for voters. Democrats had high hopes of unseating Ms. Collins after sharply outspending her in 2020 — and she still won.

And there are plenty of signs that Ms. Collins faces an uphill climb to a sixth term in a state Mr. Trump lost in 2024, including concerns about her age and the political headwinds caused by an unpopular Republican president.

But Mr. Jackson finds himself behind in a race to define himself while lacking the infrastructure or money to do so quickly. In that void, Ms. Collins and her backers will lunge to paint him in a negative light.

Democrats have other reasons to be concerned. A Supreme Court ruling late last month that rolled back decades-old restrictions on political party spending has raised worries in the party about even more spending by big donors.

“National donors are going to have to step up,” said Mr. Gifford, the Democratic fund-raiser, adding, “When you’re going to get millions of dollars spent against you, it’s extremely important to define yourself before they define you.”

Without his own super PAC, Mr. Jackson is also bound by a federal limit of $3,500 per donor per election, making it difficult for him to quickly match Ms. Collins’s numbers.

Ms. Collins has at least eight high-dollar fund-raising events scheduled from late July through December, according to a list reviewed by The Times, including events in Chicago, Nantucket and the Hamptons, and even a birthday reception a month after Election Day.

Some Maine Democrats have speculated behind the scenes about ways of filling the gap, but the options may be limited. Sara Gideon, the Democratic nominee who outspent Ms. Collins in 2020, still has nearly $2.5 million in her campaign account. Mr. Platner, despite being one of the party’s strongest online fund-raisers, had just $1.7 million in the bank as of June 30.

Neither Ms. Gideon nor Mr. Platner has said anything publicly about transferring their money to Mr. Jackson, and neither answered questions from The Times about their plans.

Mr. Jackson and his allies are projecting confidence that the high stakes of the Maine race will motivate donors.

“I’m sure everybody and their brother is going to be trying to jump into this,” he told The Times this week. “And we’re going to just do everything we can to put forward what we’re hoping to do and get support.”

Reid J. Epstein contributed reporting.