Some of Mitt Romney’s biggest supporters will remain anonymous until Jan. 31, after voters in four states have cast their votes among the Republican presidential contenders.

An organization that is spending millions of dollars to back Romney has changed the schedule it will follow to report donations to the federal government, a step that means it can postpone disclosing the names of its financial backers until after the Florida primary on the last day of January.

The group, known as Restore Our Future, is one of the largest of the “super PACs” supporting presidential candidates and could set an example for several others in its filing schedule.

The group filed a report in July revealing that it raised $12.2 million from large donors, which it named, including four contributions of $1 million. The group can accept checks of any amount under new campaign rules that went into effect last year.

Interest groups active in the campaign can choose how often they will file disclosure reports — monthly, quarterly or, in non-election years, every six months.

But those that choose to file quarterly or semiannually are also required to file a special report revealing their finances and donors if they run ads ahead of a caucus or primary.

Restore Our Future announced a $3.1 million ad campaign backing Romney in Iowa this month, making the group obligated to file a disclosure report on Dec. 22, which is 12 days before the Iowa caucus. But Restore Our Future notified the Federal Election Commission Saturday that it was switching to a monthly schedule, therefore avoiding the requirement for a December report.

Charles Spies, treasurer for Restore Our Future, said the change was made to avoid having to file reports nearly every week during the tight primary schedule.

“Unlike Barack Obama’s supporters who have been attacking Mitt Romney, Restore Our Future is fully disclosing donors,” he said.

Trevor Potter, a Republican lawyer and an advocate of tighter regulation of money in politics, said the group is circumventing a key principle of campaign law by not letting voters learn the identities of financial supporters.

“The whole premise of our system is that it’s useful for people to know who is paying for these ads and where the money is coming from,” said Potter, president of the Campaign Legal Center.

Two other Republican campaign lawyers said disclosure rules were probably not the only reason the group changed its filing schedule.

“It’s probably a dual incentive,” said Jason Torchinsky, a lawyer who worked for then-President George W. Bush. “Any PAC that’s active in primaries is better off filing monthly anyway.”

The Federal Election Commission has yet to write regulations governing the conduct of super PACs such as Restore Our Future, many of which are especially active in this year’s campaign.