A new study shows that independent campaign spending in this year's congressional campaigns by organizations other than political parties increased 73 percent over the same period in 2008.

But the authors of the study by the nonprofit Campaign Finance Institute say it's too early to tell if the U.S. Supreme Court's ruling in Citizens United contributed significantly to the increase.

The study notes that such spending approached $150 million in mid-October, with groups supporting Republicans outspending those supporting Democrats by more than 2-to-1. The former spent $105.5 million and the latter $42 million.

Campaign Finance Institute officials say the increase could simply reflect a trend that was noted before the high court ruled earlier this year in Citizens United. The ruling struck down long-time restrictions on corporate and union spending in federal campaigns.

A great deal of campaign-related spending is not reported to the FEC because it does not fit the regulatory definitions that require reporting - such as independent spending that does not include "express advocacy" for or against candidates and some targeted broadcast ads that aren't aired during the final weeks before an election. The institute estimates that reportable and non-reportable independent campaign spending for this year's congressional races will reach $560 million -- a 40 percent increase over what the CFI estimated for all independent campaign-related spending two years ago.

See the Campaign Finance Institute report here.