Just when it seemed there was no limit to what the Koch brothers and others could spend this year to influence the outcome of Senate races, the Supreme Court may rule any day in a case that could blow the lid off the limits on individual contributions to congressional races.

It has been four years since the High Court ruled that the First Amendment prohibits the government from restricting political independent expenditures by corporations, associations or labor unions. That decision paved the way for creation of conservative super PACs like the Koch brothers’ Americans for Prosperity and liberal super PACs like Senate Majority Pac that have poured hundreds of millions into presidential and congressional campaigns.

Now the justices will determine whether federal election law can constitutionally limit the amount that any one donor can give to candidates or parties in an election cycle.

Related: Clash of the Billionaire Titans

If the court strikes down those federal limits in the case of McCutcheon vs. the Federal Election Commission, it would free wealthy donors to give millions of dollars more to candidates and political parties than they currently contribute. It would represent one more giant step towards allowing the nation’s financial elites to dictate the outcome of the elections.

“It could be a game changer,” said Sheila Krumholz, the executive director of the Center for Responsive Politics. “We could see the proliferation of PACs to scoop up the money that a new tier of elite donors is able and willing to put forward, if [the court] decides to remove the aggregate limits.”

“And that of course would have an impact this cycle,” she added.

Source: OpenSecrets.Org

With control of the Senate very much at stake this year, the mid-term campaign already is awash in unprecedented sums of campaign contributions, with much of it coming from Americans for Prosperity, a group founded by the conservative billionaire brothers, Charles and David Koch.

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So far those heirs to an oil fortune have pumped $8.2 million into TV, radio and digital ads in North Carolina in a bid to unseat freshman Democratic Sen. Kay Hagan, and their group is on pace to spend more than $27 million by election day in the Tar Heel state, according to Politico. So far in this cycle, Americans for Prosperity has spent more than $12 million in the country’s half-dozen closest Senate races – or two thirds of total money spent by GOP groups nationally.

Related: Obamacare Mess Defines GOP 2014 Campaign Strategy

Republicans are voicing growing confidence they will recapture control of the Senate in November for the first time in eight years, encouraged by President Obama’s unpopularity and widespread displeasure with the Affordable Care Act, his signature health care legislation.

The aftermath of Obamacare's disastrous rollout has left Democrats vulnerable to Republican challengers who have already spent millions of dollars linking them to the president and his health care law.

Indeed, Hagen, who held a solid lead among her GOP challengers in September, began trailing two of the five Republican candidates in November, two months after Obamacare went live. By January, Hagen was behind all but one of her opponents, according to Public Policy Polling.

Likewise, Democratic Sen. Mark Pryor of Arkansas is also in trouble--thanks to Obamacare--since more than half of Arkansas voters have an unfavorable view of the law. Since January, he has trailed his challenger, Rep. Tom Cotton, by seven points.

Democrats and their allies currently control the Senate by a margin of 55 to 45. In order to win back control of the Senate, Republicans need a net pick up of six seats. Both parties consider at least 11 races competitive.

Here are seven of the most competitive races:

Kentucky – Though Senate Minority Leader Mitch McConnell leads his primary challenger Matt Bevin, conservative groups like the Senate Conservatives Fund have poured nearly $1 million into unseating the six-term senator. McConnell is expected to win the primary, but he faces a tough general election challenger, Democrat Alison Lundergan Grimes, Kentucky's Secretary of State. Recent polls show Grimes leading McConnell 46 percent to 42 percent.

Arkansas – Sen. Mark Pryor is considered the most vulnerable Democrat in the Senate. He currently trails his GOP challenger, Rep. Tom Cotton, by seven points, according to a December poll. Cotton is backed by Sen. Marco Rubio's (R-FL) leadership PAC.

North Carolina -- Five GOP candidates are vying for the chance to challenge the Democratic incumbent, Kay Hagen. The most prominent Republican in the race is State House Speaker Thom Tillis, who leads the other GOP candidates by at least seven points. Since January, Hagen has trailed all but one of those Republicans. Tillis has received most of his money from political action committees in North Carolina, including Grow NC Strong, backed by two local LLC's. But prominent conservatives, including Karl Rove and McConnell have publically endorsed and held fundraisers for him.​

Georgia: The race to replace retiring Sen. Saxby Chambliss is considered a must win for Republicans if they want to get a majority. However, polls show that the Democratic candidate, Michelle Nunn, daughter of former Georgia Sen. Sam Nunn, is enjoying a steady lead among the eight Republican candidates -- including Reps. Phil Gingrey, Jack Kingston and Paul Broun and former Georgia Secretary of State Karen Handel. Nunn has raised the most cash—a cool $3.3 million – and has received backing from several Republicans including former Senators John Warner of Virginia Richard Lugar of Indiana.

Iowa- The race to succeed retiring Democratic Sen. Tom Harkin is neck and neck between Democratic Rep. Bruce Braley and Republican Mark Jacobs. According to The Hill, Jacobs has largely funded his own campaign. With $900,000 cash on hand, he has outpaced the other GOP challengers in fundraising. The Republican National Committee has already poured money into radio ads attacking Braley over Obamacare. Braley has more than $2.3 million in his war chest from about 4,500 Iowa donors, according to The Week. ​

Louisiana – Sen. Mary Landrieu is nearly tied with her main Republican challenger, Rep. Bill Cassidy. Since August, Landrieu's approval ratings have plummeted after she was pummeled by conservative groups over Obamacare, according to The Hill. She has also been a target of the Judicial Crisis Network, which ran an advertisement slamming her for her support of Obama's judicial nominees. ​

Alaska -- Democratic Sen. Mark Begich is just barely leading his top GOP challenger, former Natural Resources Commissioner Dan Sullivan. Sullivan and the other GOP candidates continue to lambaste Begich for his support of Obamacare. And the Koch brothers’ Americans for Prosperity ran a television ad last fall blaming Begich for insurance premiums rising under ObamaCare. Sullivan raised $1.2 million last year, with most of it coming from donors outside the state, according to the National Journal.

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