WASHINGTON - Candidates, parties and outside groups have already spent hundreds of millions of dollars on political advertisements aimed at swaying the Nov. 2 congressional elections, a study found. Ad spending on House and Senate races climbed to $220 million through Sept. 15, from $135 million at the same point in 2008, according to data analyzed by the Wesleyan Media Project. The increase mainly comes from Senate races, with Florida, Pennsylvania and Nevada topping the list, the study found. 'Both Republicans and Democrats have a legitimate shot of having majorities in Congress,' spurring spending, said Travis Ridout, co-director of the project, in a statement Monday. Americans' dissatisfaction with the economy and the direction of the country is jeopardizing Democratic control of the House and Senate. Opposition to President Barack Obama has also fueled an increase in spending by Republican-leaning outside groups. Outside groups such as Crossroads GPS, advised by Republican strategist Karl Rove, have spent more than $31 million on ads for this year's Senate races, up from $16 million in 2008, the Wesleyan Media Project found. Candidates in those races have more than doubled their spending to $121 million. Some of the increase in non-candidate spending can be traced to the Supreme Court's January decision that freed corporations and unions to spend unlimited amounts from their treasuries, said Ridout, a political science professor at Washington State University in Pullman, Wash. That decision, known as Citizens United, isn't causing many corporations to do direct advertising, said Erika Franklin Fowler, another co-director of the project. Instead, more money is flowing into issue advocacy groups that don't have to report contributors to the Federal Election Commission, she said. 'We are seeing evidence of changing tactics as groups seek shelter in the rules for nonprofits that allow such organizations to withhold their donor names,' said Fowler, a government professor at Wesleyan University in Middletown, Conn. The advertising project, which used data from Kantar Media's Campaign Media Analysis Group, found that almost $350 million has been spent on advertising for gubernatorial races this cycle. About one quarter, or $89 million, came from California Republican candidate Meg Whitman, the former chief executive officer of EBay Inc. The Republican Governors Association has spent more than $12 million on ads, and Republican interest groups have outspent Democratic counterparts by a margin of 2-1 overall, the study found. Two Republican-leaning groups, Let's Get to Work and the U.S. Chamber of Commerce, have each spent close to $7 million. The top two outside groups for Democrats are California Working Families for Jerry Brown for Governor, which has spent $6.6 million, and the American Federation of State, County and Municipal Employees union, which has spent just over $6 million, barely topping Crossroads GPS. Republicans are depending on outside groups to help them overcome a deficit in official party fundraising. The three major Democratic Party committees reported a combined cash advantage of more than $20 million over their Republican counterparts heading into September. 