With a little more than a month remaining until the general election, the steady stream of cash flowing from big donors to competitive campaigns is starting to turn into a flood. Total outside spending has topped $330 million.

So for reporters keeping tabs on who’s trying to influence the vote, or citizens trying to make informed decisions at the ballot box, here’s a refresher on how to follow the money with Sunlight’s Real-Time Federal Campaign Finance tracker.

A review of the top ten independent spenders shows that some of the most powerful ‘outside’ players have deep ties to party establishments. The highest roller to date, for example, the liberal Senate Majority PAC, has fundraised in concert with Senate Majority Leader Harry Reid, D-Nev., and is controlled by a core of his longtime aides.

Other examples on our list: Left-wing dark money outfit Patriot Majority USA, helped by longtime Democratic strategist Craig Varoga, and the conservative super PAC American Crossroads, founded by former President George W. Bush’s strategist and deputy chief of staff, Karl Rove.

Both Senate Majority and Crossroads fundraise from a small pool of individual donors and corporations, but we have also seen plenty of PACs that are fueled solely by one individual or family. Sunlight has reported on the multimillion dollar cash infusion that Tom Steyer and Michael Bloomberg have devoted to their own legislative goals, and the Ricketts family of Chicago Cubs fame is the money ball behind the conservative Ending Spending Action Fund.

Biggest outside spenders

Click on the image to see the full list of outside spenders on Real-Time Influence Explorer

To navigate to our list of outside players, visit the Real-Time landing page.

Click on “Outside Spending” then “all outside spending groups,” in blue.

Unfortunately, we can only total what is reported to the FEC. Many groups taking an active role in federal campaigns register as nonprofits rather than political committees to avoid disclosing their donors. These ‘dark money’ operations, as we’ve dubbed them at Sunlight, report to the FEC only when they make expenditures urging viewers to vote for or against a federal candidate — or if they run an ad mentioning a candidate for office within a few weeks of the election.

Many spend millions on ads that run outside of the reporting window and stop just short of using the magic words. For an example of this type of “non-campaign” campaign advertising, check out the American Chemistry Council’s efforts on behalf of well-placed incumbents.

Even so, the independent expenditures that dark money groups do report to the FEC at this time of year, although just a fraction of the full scope of their political investment, do serve as a useful indicator of these groups’ activity.

Top-spending dark money groups

Among the dark money spenders, the U.S. Chamber of Commerce, the powerful trade group and ally to incumbent conservatives, has shelled out over $24 million helping sitting Republicans as stare down on a slew of antiestablishment challengers in the primary elections.

The Chamber has been a mainstay figure on the federal campaign trail for years, and although it isn’t legally compelled to disclose its donors, there’s little mystery about what interests it represents.

The same can not be said about some of the other fly-by-night nonprofit groups that sprang to life just in time for the midterm elections.

The Kentucky Opportunity Coalition, for example, is organized under section 501(c)4 of the tax code as a social welfare nonprofit group, but has shuttled over $7.5 million into the Kentucky Senate race, attacking Democrat Allison Lundergan Grimes, who’s attempting to unseat Senate Republican Leader Mitch McConnell.

As Paul Blumenthal of the Huffington Post reported in March, the Coalition shares a consultant with a McConnell super PAC, Kentuckians for Strong leadership, Scott Jennings, a former campaign adviser to the Kentucky Republican.

In another hot Senate race, Oklahoma’s Republican primary, the previously unknown Oklahomans for a Conservative Future appeared on the scene with the singular purpose of boosting state House Speaker T.W. Shannon to win the nomination to replace retiring Republican Sen. Tom Coburn. As Sunlight reported in March, the group, founded by a state lobbyist and an executive at a construction firm that performs state contracts, initially registered as a limited liability corporation in Oklahoma before later retracting that registration and organizing as a nonprofit.

To see the latest independent expenditures made by 501(c)4’s, trade associations and other politically active groups that don’t make regular reports to the FEC: On the Real-Time FEC landing page select “Non Committees” on the dropdown menu next to “Committee type.” Or, on the outside spending committees summaries page and sort by type.

Want to dig in further? Contact the Sunlight Reporting group or download bulk data from Real-Time FEC.