A Texas oil man, a Wall Street financier and several former U.S. ambassadors are among the top donors to Jeb Bush’s super PAC, providing hard evidence the Republican establishment is rallying to his presidential candidacy as he girds for a long primary battle.

Mike Fernandez, a Cuban-American billionaire who gave $3 million, is the biggest donor to the Right to Rise super PAC, which set a record fundraising pace and bested all of Mr. Bush’s rivals—Democrats and Republicans—by amassing $103 million in the first six months of 2015.

The names help confirm that the Republican establishment, supplemented with a healthy dash of Florida financial backers from Mr. Bush’s days as governor, are prepared to deliver a powerful flow of money that no other GOP candidate is likely to match.

Brad Freeman, chairman of a California private-equity firm, who gave $1 million to the super PAC and recruited other big donors, said people write seven-figure checks for different reasons. “They really believe in the candidate [or] they like to be one of the in-club,” he said. Other donors might be looking for favors, Mr. Freeman added: “For some, when it gets time to have appointments, those names stand out.”

Candidates seeking the 2016 presidential nominations disclosed their campaign’s donations and expenditures two weeks ago. But the vast majority of the money this election cycle is being raised by super PACs, independent committees operated by allies who are prohibited from coordinating their activities with the official campaigns. Mr. Bush’s super PAC is based in California and his campaign in Miami.