WASHINGTON – This has been an unusually ugly year in political advertising. Seven out of 10 ads that have aired so far in this presidential race have been negative, a huge leap from 9.1 percent in the 2008 campaign cycle, according to a new analysis from the Wesleyan University Media Project.

One big reason is the explosion of ads from independent groups, such as “super” political action committees or labor unions, that have no formal ties to candidates but are able to run ads on their behalf. In addition, project co-director Erika Franklin Fowler noted, “even the candidates’ own campaigns have taken a dramatic negative turn.”

The chief beneficiary of this onslaught has been Mitt Romney, the presumptive Republican presidential nominee.

He and his supporters won key victories in Florida, Michigan and elsewhere after blanketing the airwaves with ads questioning the credentials, records and competence of rivals Newt Gingrich, a former speaker of the House of Representatives, and Rick Santorum, a former U.S. senator from Pennsylvania.

In Florida, Romney and his backers spent an estimated $15.3 million on ads before the Jan. 31 primary. Gingrich had won South Carolina’s primary 10 days earlier and was looming as a serious threat to win Florida. Gingrich and his backers spent $3.4 million in Florida.

Kantar Media, a consulting company, found that 92 percent of the ads that ran in the state were negative, which it defined as ads that focus on the opponent of the candidate backed by the ad’s sponsor.

The tactics appear to have worked. In network exit polls, 41 percent of Florida voters said ads were important for their choices — and Romney won 59 percent of their votes, compared with Gingrich’s 25 percent.

The Wesleyan study, using Kantar Media data, found that 86 percent of interest group ads, 52.5 percent of candidate ads and 2.2 percent of political party ads were negative. It studied ads from Jan. 1, 2011, until April 22, 2012, two days before the Pennsylvania primary. Santorum left the race April 10, and Gingrich suspended his campaign Wednesday. Texas U.S. Rep. Ron Paul remains an active candidate but he’s won no primaries and trails far behind Romney in delegates.

The surge of campaign ads sponsored by independent groups has been pushed by the rise of super PACs. They’ve become influential since a 2010 Supreme Court ruling allowed corporations and others to donate and spend unlimited sums in independent campaign efforts. Outside groups have sponsored 60 percent of this year’s ads, compared with 3 percent at this point four years ago.

In Florida, Romney and his backers ran ads mocking Gingrich’s assertion that he was a well-paid historian for mortgage titan Freddie Mac and his effort to tie himself to President Ronald Reagan. Another ad showed a clip of Tom Brokaw from 1997, when he anchored “NBC Nightly News,” reporting on the overwhelming House vote to reprimand Gingrich for ethical lapses.

Negativity is thought to work in several ways, not all of them beneficial to the winning candidate.

“It can have a real negative effect on turnout,” said Kenn Venit, a media consultant in Hamden, Conn.