One aspect of the 2010 midterm elections will be all but settled by the time polls open today: The Supreme Court’s decision earlier this year to throw out political spending restrictions has dramatically expanded the flow of money in federal elections.

‘We’re seeing a sea change in American politics unfolding here,’ said Fred Wertheimer, a campaign finance reform advocate and president of Democracy 21.

As of Monday, outside groups had spent $294.4 million in the runup to Election Day, more than every other midterm cycle since 1990 combined, according to the Center for Responsive Politics’ OpenSecrets.org. This cycle’s price tag is also just shy of spending totals for unions and politically minded nonprofit organizations during the 2008 election cycle, when a protracted presidential campaign pushed outside spending to $301.7 million.

Party committees aside, the U.S. Chamber of Commerce has led all spenders this cycle, dumping more than $30 million into Congressional races across the country, including more than $3 million on the Senate races in Illinois, Washington and Colorado in the past two weeks, according to OpenSecrets.org.

[IMGCAP(1)]But the chamber’s 2009-10 outlays are a marked difference from predictions made earlier in the cycle by President Tom Donohue, who said his group was prepared to spend $75 million on Congressional races. Still, the business lobby has bet big in expensive California, spending $1.6 million in the past month to help former Hewlett-Packard CEO Carly Fiorina (R) against Sen. Barbara Boxer (D).

The business group spent the most in the past month, almost $1.7 million, in Illinois on behalf of Rep. Mark Kirk (R), who is in a tight race against Alexi Giannoulias (D).

Vulnerable House Democrats have also been targeted by the chamber, with its biggest target ‘ $550,000 in the past month ‘ being Rep. Dina Titus (Nev.), who is in a competitive race. The chamber also spent almost $443,000 against Rep. Tom Perriello (Va.), who is considered one of the most endangered incumbents and for whom President Barack Obama campaigned last week.

New groups led by Rob Collins, ex-chief of staff to Minority Whip Eric Cantor (R-Va.), and George W. Bush advisers Karl Rove and Ed Gillespie have been right behind the chamber this cycle. Rove’s and Gillespie’s groups, American Crossroads and Crossroads GPS, have spent a combined $38.5 million so far this cycle, while Collins’ American Action Network had spent $26.6 million as of Monday, according to OpenSecrets.org.

After the elections, Collins said Monday in an interview, ‘we don’t view our role as changing’ at his organization, whose mission is to ‘create, encourage and promote center-right policies based on the principles of freedom, limited government, American exceptionalism and strong national security,’ according to its website.

‘We’ve been in a role where we’re trying to educate folks as aggressively as possible, and we’ll continue to do that,’ Collins said. ‘Our methods may change a little bit, but we’ve always been very policy-focused and will continue to do so.’

Led by labor unions, Democratic-leaning groups also were taking full advantage of January’s Citizens United v. Federal Election Commission decision, which threw out most restrictions on direct political advertising paid for by corporations, trade associations and other nonprofit organizations. Under the old rules, these outside groups were prohibited from spending directly out of their general treasuries on political causes.

According to OpenSecrets.org, the Service Employees International Union has spent more than $15.7 million on political advertising this cycle, including $225,000 on recent ads to help Senate Majority Leader Harry Reid (D) in Nevada. The American Federation of State County and Municipal Employees also has come to Reid’s rescue in recent weeks, spending $300,000 on television ads that criticize his GOP opponent, Sharron Angle.

AFSCME has spent $12.4 million overall on Senate and House races this cycle, OpenSecrets.org’s data show.

While GOPers appear to have the edge with flush third-party groups, Democrats have dominated political party fundraising this cycle. The Democratic National Committee has raised $195.3 million so far this cycle, while the Republican National Committee has brought in $170.3 million, FEC records show.

The Democratic Congressional Campaign Committee has raised $129.1 million so far, while the National Republican Congressional Committee’s total was roughly $107 million. The Democratic Senatorial Campaign Committee’s overall take has been $107.2 million, while the National Republican Senatorial Committee trailed with $92.9 million.

‘They have done a great job filling the massive void that the RNC left open,’ a Republican strategist said of the impact of outside organizations this cycle. ‘Liberal groups continue to outspend conservative-allied groups, but their efforts have helped close the gap and complement the recent fundraising surge that Republicans have seen at the campaign committee level.’

But the excitement among some Republicans ‘ particularly Members ‘ may be short-lived. The explosion of spending by outside groups is seen by many election law experts as coming at the expense of party committees, which must still raise money under strict fundraising guidelines and disclose their donors.

‘Virtually every entity in America is unlimited expect party committees,’ Republican campaign finance lawyer Michael Toner said Monday. ‘There’s going to be a hard look at where political parties sit.’

Alex Knott contributed to this report.