In September, outside spending crushed all past months on record, at over $200 million, according to Sunlight’s Follow the Unlimited Money tracker, and prior years' independent expenditure filings with the Center for Responsive Politics.

To put the September figure in perspective, the $207 million is more than double what was spent in August. It represents 38 percent of all of the $548 million in independent expenditures in the 2012 election and about two thirds of the total outside spending in the entire 2008 election, according to totals compiled by CRP. That year also featured a presidential race — the last one before the 2010 Citizens United decision that opened the floodgates for outside money.

Nor is it likely we've captured it all: Keep in mind that both Sunlight and CRP rely on spending reported to the Federal Election Commission–there have been millions of dollars spent by groups over the years that do not disclose their spending to the FEC.

In this election, the effects of the Supreme Court's Citizens United decision and other court rulings, which allowed outside groups to spend an uncapped amount on elections from unrestricted sources, now seem to be in full bloom. In 2010, groups were still catching up to the law, and the first super PACs were only formed a few months before the election.

About one-third of the $207 million spent in September is provided by unknown donors, leaving voters without an idea of who is spending thousands or millions of dollars to help their candidate of choice win a seat. That is because this money comes from 501(c) nonprofit groups—what we are calling “noncommittees” in the chart to the left; these groups seldom disclose their donors to the public or even register with the Federal Election Commission. Another third comes from super PACs, which must disclose donors, and the last third comes from party committees and traditional political action committees, which face limits on how much they can raise from each donor.

October is on pace to just about match the huge September influx, with nearly $12 million already spent in the month’s first two days. And the total for the first two days of October is likely to go up: Filings for the first two days of the month are still being reported to the FEC, where groups are required to file notice within 24 to 48 hours of an independent expenditure for a political campaign.

Case in point: On Tuesday, Politico reported that GOP-aligned American Crossroads and Crossroads GPS just unleashed its biggest media buy this cycle: $16 million in TV and radio ads attacking President Obama and four Democratic senators in tight races.