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.