In theory, Democrats should be poised to play offense in a Senate election where they must hold onto only 10 seats while Republicans defend 24. But a huge, early spending blitz by GOP party committees, candidates and outside groups may upend expectations in contested states from Illinois to Nevada.

Spats over campaign financing have also taken center stage in several Senate contests, as candidates trade accusations of breaking the election laws and challenge one another to bar outside giving in their races.

Money isn’t the only variable that will decide who wins the Senate next year, of course. But the relatively small number of seats in play — coupled with a Senate map that will feature hard-fought contests in such key presidential states as Florida, Ohio and Pennsylvania — guarantees a race awash in money.

The cost and volume of TV ads will be sky high. And in Senate races, as on the presidential campaign trail, the already-flimsy political money rules have been blown apart by billionaire donors and freewheeling outside groups.

“You will see wealthy donors deciding that they can have a very big bang for their buck when they go after these Senate races,” says Lawrence Noble, senior counsel at the Campaign Legal Center, a nonpartisan legal institute. “Money can’t guarantee you an election, as we’ve seen in some very big races. But you’re much better off with the very big money than you are without it.”

Republicans set the tone early with a spending onslaught that has already hit $14.7 million, swamping Democratic expenditures and far surpassing GOP spending at this point in the 2014 midterms. GOP early spending is coming from candidates, party committees and special-interest allies.

In Ohio alone, Americans for Prosperity, the conservative advocacy group underwritten by billionaire industrialists Charles and David Koch, has spent close to $1.5 million assailing former Gov. Ted Strickland. Strickland is contending in the Democratic primary with Cincinnati City Councilman P.G. Sittenfeld in a bid to challenge Republican Sen. Rob Portman, but the more-seasoned Strickland is viewed as more formidable.

The U.S. Chamber of Commerce, which has announced plans to spend as much as $100 million in this election, has also shelled out more than $500,000 in attacks on Strickland. Overall, GOP spending in Ohio tops $5.5 million, Democrats estimate.

“The sheer amount of dollars that billionaires like the Koch brothers and outside special interests are spending on Republican incumbents surpasses anything we’ve ever seen before,” says Justin Barasky, outgoing communications director for the Democratic Senatorial Campaign Committee.

Democrats contend that big GOP spending shows that Republicans are running scared. But Democrats and their allies have also spent heavily in such states as Illinois, where Mark S. Kirk is considered one of the most vulnerable Senate Republicans. Kirk, who suffered a stroke in 2012, will likely face either Democratic Rep. Tammy Duckworth or her primary opponent, Andrea Zopp, former president of the Chicago Urban League.

Targeted by close to $1 million in negative ads from such outside groups as the Natural Resources Defense Council, Kirk appealed to voters directly in a $300,000 ad buy that testified how struggling Illinois families and American soldiers inspired him as he climbed the Capitol steps to recovery.

“We were very concerned about outside groups coming and trying to define the senator,” says Kirk campaign manager Kevin Artl. “So we made the decision that we were going to get out there early with an ad that talked about Mark’s recovery from the stroke, and that defined him before others could define us.”

Unleashed by the Supreme Court’s 2010 ruling to deregulate political spending, super PACs may raise unlimited contributions only if they keep candidates at arm’s length. But the firewall between such PACs and the parties and candidates they back is evaporating.

The Senate Leadership Fund, a new super PAC to defend the GOP’s Senate majority, has raised $5 million since January. An allied advocacy group dubbed One Nation has spent $2 million to help GOP Senate candidates in Illinois, New Hampshire, North Carolina, Ohio and Pennsylvania.

Also taking off in this election are super PACs that shower money on a single candidate. Already common in the presidential contest, such PACs are also raising millions to back Senate candidates in Florida, Ohio and Wisconsin.

“We are seeing these ‘buddy PACs’ that are closely affiliated with the candidates, to the point of raising the question of whether the candidate controls that group,” says Ian Vandewalker, counsel in the Democracy Program at the Brennan Center for Justice at New York University’s School of Law.

In Wisconsin, a super PAC dubbed Restoration PAC has spent $750,000 in attacking former Democratic Sen. Russell Feingold, who is seeking to win back his old Senate seat from Republican Ron Johnson. In Ohio, a super PAC backing Portman dubbed Fighting for Ohio has raised $186,000 from four donors.

In the open-seat contest to succeed Florida Republican Sen. Marco Rubio, who is running for president, a super PAC dubbed the Service, Honor, Country Action Fund has rallied behind GOP House member and Senate hopeful Ron DeSantis.

In a crowded Florida GOP primary, DeSantis has won the endorsement of both conservative activists and party leaders. Another GOP primary contender, Florida Lt. Gov. Carlos Lopez-Cantera, launched and raised money for his own super PAC, known as Reform Washington, before announcing his candidacy. Lopez-Cantera now serves as the super PAC’s honorary chairman.

Democrats have complained about such cozy arrangements. But lawyers for the Senate Majority PAC recently signaled that Democrats may soon take a page out of the GOP’s book. They complained to the Federal Election Commission that GOP spending violates the rules, but also sought FEC permission to follow suit.

Mad Money

In several races, political spending has emerged as an issue in the contest. In Wisconsin, Johnson has sought to paint Feingold as a campaign-finance hypocrite. A longtime champion of political money curbs, Feingold co-authored the 2002 ban on unrestricted “soft” money raised by political parties.

Johnson has slammed Feingold for launching a leadership PAC that did little to help candidates; for accepting speaking fees from private institutions, a practice he once criticized; and for abandoning his previous pledge while senator to collect most of his money in Wisconsin.

The DSCC and Wisconsin Democrats have countered with an ad that raises questions about $10 million in “deferred compensation” that Johnson received from his plastics company, Pacur, after personally spending $9 million on his 2010 Senate race. Feingold has also challenged Johnson to place a voluntary moratorium on out-of-state spending in the race.

In Florida, Rep. Alan Grayson has also challenged his Democratic primary rival and fellow House member Patrick Murphy to take a similar pledge. Maryland Democratic Rep. Chris Van Hollen has done the same in his primary against House colleague Donna Edwards in the open seat race to succeed outgoing Democratic Sen. Barbara A. Mikulski. So far such challenges have yet to trigger an agreement.

Like the presidential candidates who have launched attacks on Washington special interests, Feingold, Grayson and Van Hollen are responding to an angry electorate increasingly suspicious of politicians and the big donors behind them. But on both sides of the aisle, political players say they will not unilaterally disarm — a guarantee that spending will continue to soar as the rules fall away.

“I think that voters believe that it is a problem,” says Rodell Mollineau, a partner with the consulting firm Rokk Solutions who formerly headed a Democratic super PAC. “But I have yet to see voters believing that it is so much of a problem that they are going to vote on it, or punish anyone for it.”