And knocking out Bush as the economic elite's favorite candidate is a main goal of the Rubio effort. But if he succeeds, Rubio could find himself in the same dilemma as Bush: being the establishment candidate in a party that at the moment loathes the establishment.
Rubio already had a big problem on immigration, though his relatively low standing in the polls masked it from becoming an existential crisis for his campaign. He ran for the Senate as an opponent of "amnesty" for the 11 million undocumented immigrants in the U.S., then championed an immigration reform package with the "Gang of Eight" in the Senate that included a path to legal status that the right regards as amnesty.
Rubio has since disowned his own immigration reform bill though more on procedural than substantive grounds. But with Singer's backing, Rubio will now open himself up to major attacks from Donald Trump and the other strongly anti-immigration voices in the Republican Party. They will be able to more effectively rip him as favoring the elite view of the GOP that wants comprehensive reform with a path to citizenship rather than mass deportations.
Rubio's views on immigration could serve him well in a general election campaign — if he can actually make a clear case for what they are — but they are potentially devastating in the Republican primaries. This is partly why the Rubio campaign has hoped to continue to fly slightly below radar and not peak until much closer to when actual voting begins in January.
They may no longer have that luxury. Rubio is still running well behind Trump and Ben Carson but he is now clearly in third and is emerging as the establishment favorite over a stumbling Bush.
The more he rises, the more fire he will draw from Trump and the conservative media juggernauts led by Breitbart News and the Drudge Report. If conservative media start labeling Rubio as the "pro-amnesty" Wall Street candidate, it could put a serious lid on his ability to move into the top spot should Trump and Carson fade. One person who does not have that problem is Sen. Ted Cruz, who has shown a remarkable ability to hoard cash and position himself to inherit Trump's supporters.