Big money isn't just pouring into the 2016 presidential race-which is already on pace to break several campaign spending records. The campaign-finance arms race has also driven big money into several contentious U.S. Senate races much earlier than usual.

"Early money is like yeast, or so goes the saying, implying that when you raise and spend big money early, it leads to more campaign cash," says a new Sunlight Foundation report analyzing the most expensive Senate races so far this election season.

With Democrats vying to retake the Senate, several high-stakes races have already proven to be big-money magnets. Candidate committees, super PACs, and political nonprofits have spent more than $65 million to date on the 10 most expensive Senate races, according to Sunlight's analysis of FEC filings.

Those races include the Ohio battle for incumbent Republican Senator Rob Portman's seat, which is so far the biggest of the big money Senate races. Portman faces a formidable Democratic foe in Ohio's former Democratic Governor Ted Strickland. But Portman is sitting on a $12.7 million war chest, compared to Strickland who only has $2 million on hand.

He also has plenty of help from outside groups, which so far have spent $4.2 million in the race. Much of that is on behalf of Portman, with Republican super PACs and other outside groups like the Koch-funded Freedom Partners and the NRA Political Victory Fund spending millions.

The second-most expensive Senate race is in Maryland, where nearly $9 million has already been spent. Senator Barbara Mikulski's retirement has turned the Democratic primary race into a heated battle between Maryland Representatives Chris Van Hollen and Donna Edwards. The Democratic primary victor is widely expected to win the general election against the Republican candidate in the dark blue state, which makes the preliminary contest a big-money bonanza for the eventual winner.

Van Hollen has $3.6 million on hand, compared with Edwards's $300,000. But Edwards has benefited from the outside support of Emily's List's super PAC, which has so far spent $2.4 million on her behalf. The National Association of Realtors' political arm has spent nearly $1 million in support of Van Hollen.

In Wisconsin, the Republican incumbent and conservative darling Ron Johnson is running against former Wisconsin senator and progressive firebrand Russ Feingold. The Democrat has created a successful grassroots fundraising campaign, and with $4.8 million on hand, is slightly outraising Johnson.

Outside spending has been a little more than $750,000, most of it going to attacks on Feingold from the free-market conservative Club for Growth super PAC. However, Johnson has plenty of friends in, and connected to, the Koch brothers' machine, so Wisconsin may turn into an outside-spending battlefield.

The 2014 elections, which the GOP swept into control of the Senate, attracted the largest-ever amount of outside spending on Senate races. And 2016 is on pace to break that record. As the Prospect's Eliza Carney wrote earlier this week, the specter of a Trump nomination is likely to drive many big donors to spend on Senate races instead of the presidential race. The Democratic Senatorial Campaign Committee is already outraising its GOP counterpart as the prospect of a Senate takeover has produced big blue fundraising numbers early on.

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However, as the Sunlight report notes, the biggest story of the 2016 Senate races will likely be the big money that we can't see. Undisclosed spending by dark-money political nonprofits is increasing. Moreover, with the Federal Elections Commission and the Internal Revenue Service failing to scrutinize these activities, dark money groups have a green light to play fast and loose with their nonprofit tax status, which requires that they spend the majority of their funds on "social welfare."