Donald Trump has his billions, but Hillary Clinton has her billionaires.

As the candidates formally enter the general election season after their conventions, the former secretary of state’s wealthiest backers are pouring money into political groups opposing Trump. Within the past year, according to a review by FoxNews.com, a total of 24 billionaires have donated more than $42.5 million to two Clinton campaign arms and three allied super PACs.

All this is in preparation for a blitz of advertising and other efforts to defeat Trump over the next three months – as some big-money Republicans stay on the sidelines.

Hewlett Packard Enterprise CEO Meg Whitman, for instance, is joining industrial power-brokers Charles and David Koch in refusing to back fellow Republican billionaire Trump. While the Koch brothers plan to stay out of the presidential race, Whitman is going one further by stating she will endorse and help fund Clinton.

Whitman will be in some influential company. Her most famous Clinton donor colleagues include movie producers Steven Spielberg and Jeffrey Katzenberg, financiers Warren Buffett and George Soros, and Walmart heiress Alice Walton.

The billionaire donations were given to Clinton campaign funds Hillary for America and Hillary Victory Fund; and the pro-Clinton PACs American Bridge 21st Century, Priorities USA Action 2016 and Correct the Record.

The largest donor was Soros with $9 million, according to 2015 and 2016 Federal Election Commission records. This was followed up by mathematician and hedge fund manager James Simons at $7 million and Haim and Cheryl Saban, with $3.5 million each. Haim Saban owns the Spanish television network Univision.

Most of the money is donated to PACs because candidate campaigns are only allowed to accept $2,700 per donor. Clinton’s overall campaign total is $374.5 million and Trump’s is $98.7 million through July 21, according to the Center for Responsive Politics.

The Clinton campaign – which only recently put away a primary challenge from Bernie Sanders, who boasted of his small-dollar donations – stresses that the bulk of the nominee’s support is grass-roots.

"More than 900,000 people contributed to Hillary Clinton's campaign in July and the average donation is just $44,” Clinton spokesman Josh Schwerin said. “That grass-roots support is the true power behind this campaign as Hillary lays out her plans to build an economy that works for everyone, not just those at the top.”

But the high-dollar backing from some of the wealthiest Americans contrasts with some of the themes at last week’s Philadelphia convention. Clinton, who called in her convention address to get “money out of politics,” repeatedly has said her presidency would work for lower- and middle-class Americans while taxing the wealthiest, like Wall Street executives.

“I believe American corporations that have gotten so much from our country should be just as patriotic in return. Many of them are. But too many aren't,” Clinton said during her nomination acceptance speech. “It's wrong to take tax breaks with one hand and give out pink slips with the other. And I believe Wall Street can never, ever be allowed to wreck Main Street again.”

The super PACs, though, are a valued resource to push Clinton along in her quest to be the first female president.

American Bridge 21st Century was founded in 2010 to provide opposition research to the Democratic Party by following key Republicans and videotaping everything they say. The material is used to blanket airwaves and the Internet with their foibles. Its site is mostly dedicated to the presidential race.

Priorities USA Action 2016 was founded a year later with the purpose of raising funds from wealthy donors. Its website states “We’re all in for Hillary Clinton.” Correct the Record’s mission is to “defend Hillary Clinton against baseless attacks.”

Despite the looming ad onslaught, Trump’s campaign is voicing confidence about its financial position, after announcing an $80 million July fundraising haul for the campaign and the GOP. The haul marks a big fundraising surge, and comes close to the combined $90 million raked in by Clinton and the Democrats last month.

"The campaign is in good shape. We are organized. We are moving forward," campaign chairman Paul Manafort told Fox News' "Happening Now" on Wednesday.

While Trump also is willing to keep spending from his own fortune to pad the account if necessary, Clinton enjoys a deep bench with deep pockets.

The other billionaires who donated to her cause are: heir to the Hyatt Hotel chain J.B. Pritzker and his wife Mary; Slim-Fast founder Daniel Abraham; film executive Thomas Tull; entrepreneur Marc Benioff; Facebook COO Sheryl Sandberg; Jon and Pat Stryker, heirs to a self-named family medical device company; scientist David E. Shaw; philanthropist Barbara Lee; financiers John Doerr, Bernard Schwartz, Roger Altman, Henry Laufer and Herb Sandler; and LinkedIn co-founder Reid Hoffman.

Oprah Winfrey stated on television that she endorses Clinton, but she doesn’t have any donations listed on the FEC website.

But one high-profile backer has decided to fund his own anti-Trump endeavors.

Billionaire Tom Steyer spent $1.9 million through his environmental action organization NextGen to produce a television commercial titled “The Wall,” which ran in California during the Republican convention. In the commercial, Steyer and a group of young people stand in front of a montage of Trump clips where the candidate discusses building a wall at the border. “That’s not America,” Steyer states as the group says in chorus, “Vote!”

Earlier this year, Steyer told reporters he would spend $25 million in an effort to turn out youth voters. He historically has used the nonprofit NextGen as a political platform for Democratic candidates.