The casino magnate's check to the pro-Romney Restore Our Future instantly skyrockets him to the top of that super PAC's list of megadonors.

WASHINGTON — News that casino magnate Sheldon Adelson is pumping $10 million into a “super PAC” backing Republican presidential challenger Mitt Romney underscores how swiftly a single wealthy donor can affect a campaign.

Adelson’s contribution, first reported Wednesday by The Wall Street Journal, comes after he and his family pumped $21.5 million into a super PAC (political-action committee) that supported former House Speaker Newt Gingrich. The Adelson family effectively subsidized the entire Gingrich super PAC — its contributions made up 89 percent of all money raised by Winning Our Future.

Adelson now is poised to play a similar role for Romney. With Adelson’s check to Restore Our Future, he instantly skyrocketed to the top of that super PAC’s list of megadonors. In all, Restore Our Future had raised $56.5 million through the end of April, according to the Center for Responsive Politics.

The Romney campaign and Carl Forti, political director of Restore Our Future and a former Romney aide, both declined to comment. A spokesman for Adelson did not return a call seeking comment.

Romney has courted the Las Vegas Sands chief executive since Gingrich dropped out, meeting with him privately last month. Advocates of campaign-finance limits pounced on that, noting candidates are not allowed to solicit more than $5,000 from donors to super PACs, which are required to operate independently of official campaigns and political parties.

Adelson is expected to pour substantially more money into this year’s campaign. But it is unclear how many of his donations will be reported publicly, since many organizations financing campaign-related ads, such as the Karl Rove-founded Crossroads GPS, are organized as tax-exempt groups and do not disclose donors.