Winning big in the Super Tuesday states will prove increasingly difficult for Gingrich if he cannot match Romney in advertising, especially with contests spread across the country. Even with Adelson's support, Gingrich estimated that Romney doubled or quadrupled his spending in early voting states.

It is unclear why Adelson is deciding now to pull out of backing the former House Speaker, although CNN is reporting that Adelson and Romney met while the former governor was campaigning in Nevada last week. According to the network's sources, Adelson "assured Romney that he will be behind him 100 percent should he become the nominee."

Meanwhile, Reuters is reporting that the Department of Justice and Securities and Exchange Commission have shown interest in opening an investigation into Adelson's casino operations in Macau. Were Gingrich to eventually capture the nomination, the casino mogul might be attempting to insulate him from the political fallout surrounding a high-profile backer being the subject of a law enforcement investigation.

Gingrich has previously dismissed money concerns by arguing that he is running an "unconventional campaign" operation. But the apparent loss of his highest-profile — and most generous — donor could leave Gingrich limping into the Super Tuesday contests.