By Michelle Conlin and Grant SmithNEW YORK (Reuters) - Within minutes of Jeb Bush dropping out of the presidential race Saturday night, some of his donors were preparing to throw their financial support behind Marco Rubio, who has emerged as the strongest candidate among the establishment wing of the party."Jeb's network is already naturally migrating to Marco," said Gaylord Hughey, a top Bush fundraiser from Texas, echoing what four other top donors told Reuters. "It's the clear path.""It's a stampede," added another donor, who spoke on the condition of anonymity because he wanted to give Bush some time after dropping out before he went public with his support of Rubio, the U.S. senator from Florida.Three other Bush donors, who declined to be named, also said they now planned to support Rubio.Although he has failed to win any of the first three nominating contests, Rubio is considered by many political strategists as the best positioned to challenge frontrunner Donald Trump, a billionaire political outsider, and Senator Ted Cruz of Texas, who has campaigned on an anti-Washington message.The likelihood that some of Bush's deep-pocketed donors will back Rubio comes at an opportune time for his candidacy, as he heads into a series of contests in March that will be crucial for building momentum.Brian Ballard, who raised money for Bush last year but switched allegiances last summer to Rubio, said: "It's flooding tonight. Ninety-five percent of Jeb's money is going to end up with Marco."Rubio had only $5 million in cash on hand at the end of January, federal campaign finance reports released Saturday night show, a slim buffer by modern campaign standards.Conservative Solutions, a political spending group that supports his campaign, had $5.6 million on hand at the end of January, but spent an additional $9.3 million on advertising in the first 19 days of February.By contrast, Right to Rise, Bush's political spending group, had $24.5 million on hand at the end of January but spent $6.6 million of that in February on ads. And his network of donors have proven their financial might, helping Bush at one point to amass a war chest of $150 million.Even before Bush dropped out, some donors privately complained about the missteps of the campaign, expressing concern about the strength of Trump's candidacy and Bush's lackluster debate performances.In January, a dozen top Bush donors interviewed by Reuters said they were angry that Bush was using their money to lob attack ads at Rubio, damaging the candidate they thought had the best chance to win.They also said they had privately signaled to the Rubio campaign that they would support him once Bush dropped out. But they wouldn't go public at the time because they feared displeasing the Bush dynasty.In addition to the Bush money, Rubio is expected to reap the support of the billionaire industrialist Koch brothers and their donor network, which is expected to spend $400 million in the presidential race, according to several network donors and Koch political organization officials.The Koch brothers are also in the midst of planning efforts to undercut Trump's candidacy, network officials and members said at a recent private retreat in California.Their focus group research has shown that when voters learn more about how Trump's past business behavior has hurt ordinary people, they sour on him, according to the sources, who spoke on the condition of anonymity.(Reporting By Michelle Conlin; Editing by Paul Thomasch and Mary Milliken)Join the conversation about this story Click here to read full news..