The battle in Iowa has underscored what advocates for tighter campaign finance restraints have warned for months: that the new groups will be deployed to devastating effect, in the primary season and then in the general election.

“Iowa is ground zero of what we can expect in every competitive state for the rest of the presidential election,” said Ellen S. Miller, executive director of the Sunlight Foundation, which tracks outside money in politics.

Since they began advertising in earnest several weeks ago, groups like Restore Our Future have spent millions of dollars in the early primary states, rivaling and in some cases surpassing the spending of the candidates they support. While the candidates can raise just $2,500 from each individual donor for the primary, super PACs, thanks to the Supreme Court’s Citizens United decision, face no such restrictions.

Speaking on Fox News last week, Mr. Romney played down the significance of Restore Our Future’s advertisements against Mr. Gingrich, arguing that Mr. Gingrich was falling in polls as voters focused on his record. Mr. Gingrich has also been under intense assault from other groups, including Ron Paul’s campaign; Mr. Romney’s campaign itself called attention to Mr. Gingrich’s tumultuous departure from Congress in a mailing it sent to Iowa voters.

But one thing is clear: Restore Our Future has spent more on advertisements in Iowa and elsewhere than any other super PAC, according to tracking by NBC and Smart Media Group Delta. The group has already begun buying television time in two other Republican primary states, Florida and South Carolina, running ads that hammer Mr. Gingrich and Gov. Rick Perry of Texas.

A super PAC supporting Mr. Gingrich, Winning Our Future, has spent just $263,000 on advertising in Iowa, according to figures from NBC and Smart Media Group Delta, without explicitly attacking Mr. Romney. (Another pro-Gingrich group, Strong America Now, has attacked Mr. Romney in mailings to voters.) Restore Our Future has spent twice as much money in the state as Mr. Romney has, most of it on advertisements savaging his opponents. Meanwhile, Mr. Romney’s campaign has run only positive television ads, featuring sunny portrayals of him and his family, with the occasional jab at President Obama.

That has helped Mr. Romney avoid the classic conundrum of political attack advertising in a nominating battle: Negative commercials tend to harm both the candidate making the claim and the one on the receiving end. One aide said Mr. Romney has apparently suffered “no collateral” damage from Restore Our Future’s negative advertisements against Mr. Gingrich, which are not identifiably connected to Mr. Romney.