Outside groups have spent more in Colorado’s U.S. Senate race than in all but one contest nationwide, according to an analysis of independent expenditures released Thursday by a campaign-finance watchdog group.

Since the Aug. 10 primary, outside interests have spent nearly $4 million in the Senate race that pits Democrat Michael Bennet against Republican Ken Buck, according to data compiled by The Sunlight Foundation. That total trails only the $4.12 million spent by outside groups in the Pennsylvania Senate race between Republican Pat Toomey and Democrat Joe Sestak.

Since January, Bennet has benefited from $2.32 million in independent expenditures either supporting his candidacy or opposing Buck, while the Republican challenger’s total in those categories is $2.01 million, according to foundation figures compiled from recent FEC filings.

Political analyst Floyd Ciruli said the financial influence of outside groups has grown in recent elections as their donors are not hindered by contribution limits and in many cases are afforded anonymity.

“It’s a sign of the importance and winnability of this race for both sides that it’s getting so much attention,” he said of the money being spent in the Colorado Senate contest.

Among those that have plunked down the most money in the Buck-Bennet race are the Democratic Senatorial Campaign Committee, which has spent $1.67 million going after Buck; and American Crossroads, a group backed by former George W. Bush political adviser Karl Rove that has spent $1.04 million to defeat Bennet.

Nationwide, outside spending has nearly doubled since the same period in 2006.

“Spending by outside organizations to influence congressional elections totals some $57 million — up more than $25 million from a comparable time period in 2006, the most recent non-presidential election cycle — according to reports collected by the Federal Election Commission,” the foundation said in a release.