(Reuters) - U.S. oil and gas executives bet big on 2016 Republican presidential candidate Jeb Bush - they donated more to his White House run than to all of his rivals combined, according to a Reuters review of campaign disclosures.

U.S. Republican presidential candidate Jeb Bush holds a town hall meeting with employees at FN America gun manufacturers in Columbia, South Carolina in this February 16, 2016, file photo. REUTERS/Jonathan Ernst/Files

But that was last year.

Now Bush faces what party strategists and donors view as a make or break moment on Saturday in South Carolina’s Republican primary, or early nominating contest. Polls show him trailing in the single digits. If he fails to do well, there will be pressure on him to quit, strategists say, and the oil money will be looking for a new home.

The main beneficiaries in the Republican race would likely be Ted Cruz, a U.S. Senator from Texas, and Marco Rubio, a U.S. Senator from Florida. After Bush, the two candidates received the most contributions from the oil and gas industry, according to the Reuters review.

When Bush entered the White House race last year, the petroleum sector saw him as their natural choice: he was the son and brother of former presidents and he came from a West Texas family with historically close ties to the oil industry.

“Bush is part of a family that is a friendly face to the oil industry,” said Sarah Emerson, director of Energy Security Analysis Inc. in Boston.

He drew more than $2 million from the chief executive officers of companies like Exxon Mobil XOM.N, Halliburton HAL.N, Kinder Morgan KMI.N, and Chief Oil & Gas in 2015, making up about 56 percent of all the industry’s contributions to the race so far, according to the review.

The review covered contributions from 75 oil and gas companies, their employees, and their political action committees to presidential candidates’ campaigns and allied Super PACs. When counting donations only to Republican candidates’ campaigns, employees favored Cruz among the Republicans, with Bush in second place and Rubio in third.

(Click here for a graphic on oil and gas money in the U.S. presidential race: tmsnrt.rs/24cW28e )

Several of Bush’s biggest oil CEO donors have links to his family. Richard Kinder of Kinder Morgan, for example, campaigned for George H.W. Bush’s run for the White House in 1992, and for George W. Bush’s run in 2004. Halliburton’s CEO, David Lesar, meanwhile, succeeded Dick Cheney after George W. Bush nominated him vice president in 2000.

“Some of those people may be thinking support can get them a place in Bush-world,” said one Washington D.C.-based oil industry lobbyist, who asked not to be named. “The question is, where does that money go if Bush bleeds out?”

Bush is now running in fifth place among seven Republicans in a national Reuters/Ipsos poll. He has just 8 percent support leading into Saturday’s primary, compared with front-runner Donald Trump’s 40 percent. Cruz and Rubio came second and third in the poll with 17 percent and 11 percent, respectively.

Donations directly from oil companies and their political action committees or PACs have been practically nil – suggesting that millions of dollars remain on the sidelines of the race that could be deployed once the field of candidates narrows.

RISE OF THE CEO

This year’s election is particularly important to oil and gas executives because of a steep slump in energy prices that has slashed profits and worries that another Democratic president could strengthen curbs on drilling and carbon emissions introduced by President Barack Obama.

“There is a pronounced partisan agenda on energy,” said Robert McNally, founder and president of the Washington-based global energy consultancy The Rapidan Group, explaining the two parties have moved farther apart on energy policy. “There’s no middle of the road anymore,” he said.

As a group, oil and gas company CEOs have doled out nearly $3.2 million in support of presidential candidates so far – more than three times more than all oil company employees did in the equivalent period of the 2012 race. One million dollars of that went to Texas Gov. Rick Perry, who dropped out in September.

The CEOs are not alone in giving big. Some families that made their cash in the oil industry have also been major donors, like the Wilks brothers of Texas, who have given Cruz’s Super PAC $15 million.

The biggest oil company CEO donations to current candidates have come from the heads of Kinder Morgan and Chief Oil and Gas, at more than $1 million each to Bush's campaign and his allied Right To Rise USA Super PAC, according to the filings to the Federal Election Commission. Rubio, meanwhile, took some $38,000 from chief executives at Devon Energy DVN.N, AGL AGL.AX, and Atmos ATO.N. Cruz received no money from energy company CEOs.

Individual donations from oil company employees who were not CEOs appeared to favor Democratic hopeful Hillary Clinton, who took in more than $110,000. Cruz was next with nearly $105,000.