New rules requiring greater online disclosure of political TV advertisements in the country's top 50 markets wouldn't have applied to the majority of ads aired by President Obama's campaign and by his chief supporters and detractors during an 11-day period in April, according to a new analysis.

In a report released Thursday, the Wesleyan Media Project found that TV advertising in the presidential race has gotten a lot nastier than it was in 2008. But the report also recorded the TV markets in which three heavyweight political groups aired ads supporting presidential candidates between April 11 and 22. Altogether the groups — the Obama campaign, Priorities USA, a super PAC supporting Obama, and Crossroads GPS, a Republican political nonprofit headed by Karl Rove — aired 3,299 of 6,095 ads in smaller markets.

The big groups' focus on smaller media markets show the limited reach of a new Federal Communications Commission rule that will require TV stations in only the top 50 markets to post their political advertising files online this year. About 68 percent of US residents live within the top 50 TV markets, but large swathes of important battleground states lie outside of them. Of the ads that the Obama campaign and the two big super PACs aired during the April period that the Wesleyan project tracked, 54 percent were not in markets covered by the new FCC rule.

The new rule on online disclosure also only applies to ads aired on stations affiliated with the top four broadcast networks, so the actual number of ads that would have had to be disclosed could be significantly lower. The Wesleyan analysis relies on data from Kantar Media/CMAG that includes ads made on all broadcast channels and national cable networks. The data isn't broken down by network.

The Wesleyan media group looked at ads that aired after Rick Santorum dropped out of the race. "It's a window into general election spending now that for all intents and purposes the primary election has wrapped up," said Erika Franklin Fowler, assistant professor of government at Wesleyan University. It's not clear how much those ads cost to run, or how many saw them.

Since 1938, broadcasters have been required to make political candidates' TV spending available, but until a vote last month there was no requirement that this information be put online. Media and government transparency groups had long argued that paper disclosures only available at local TV stations were an anachronism that left the public largely in the dark. But TV broadcasters' groups have opposed online disclosure, arguing against both digitization cost and publication of the rates they charged political candidates, which by law are the lowest they offer. In 2007 the FCC voted down a rule that would have required broadcasters to host the political file on their own web sites.

On April 27 the FCC modified its stance, voting to approve a compromise requiring only online disclosure from broadcasters affiliated with the four biggest networks and broadcasting in the 50 biggest markets.