The Crossroads ad buys are part of a wave of spending from conservative groups in the second half of September that swamped liberal outfits, a POLITICO analysis of federal data indicates.

And they’re not slowing down. The two groups are about to launch a week-long advertising campaign in key battleground states worth $16 million. That includes $11 million on a spot focusing on the stimulus bill and the unemployment rate.

Top-spending liberal organizations such as House Majority PAC and Priorities USA Action simply couldn’t compete last month, as has been the case for most of the 2012 campaign. Of the 10 groups spending the most in late September to directly support or oppose federal candidates, $47.4 million came from identifiably conservative entities while liberal groups spent $21.8 million, records show.

Here’s the rundown on the Top 10 outside political groups, from Sept. 15 to Sept. 30, sponsoring ads and messages known as independent expenditures that overtly support or oppose federal political candidates:

1) U.S. Chamber of Commerce, $11.3 million, primarily in opposition to Democratic congressional candidates.

2) American Crossroads, $10.97 million, primarily in opposition to Obama and Sen. Bill Nelson (D-Fla.).

3) Crossroads GPS, $9.16 million, primarily in opposition to Obama and a variety of Democratic Senate candidates, including Sens. Sherrod Brown (D-Ohio) and Jon Tester (D-Mont.) and Reps. Tammy Baldwin (D-Wis.) and Joe Donnelly (D-Ind.).

4) Americans for Job Security, $8.23 million, all in opposition to Obama.

5) House Majority PAC, $6.73 million, primarily in opposition to numerous Republican House candidates, including Reps. Jim Renacci (R-Ohio), Jeff Denham (R-Calif.), Allen West (R-Fla.), Charlie Bass (R-N.H.), Nan Hayworth (R-N.Y.) and candidate Vernon Parker of Arizona.

6) Priorities USA Action, $6.19 million, all in opposition to Mitt Romney.

7) American Federation of State, County and Municipal Employees, $4.94 million, all in opposition to Romney and numerous Republican congressional candidates including Rep. Dan Lungren (R-Calif.), Chip Cravaack (R-Minn.), Senate candidate Linda McMahon of Connecticut, Hayworth and Renacci.

8) Restore Our Future, $4.14 million, all in opposition to Obama.

9) Service Employees International Union committees, $4 million, in support of Obama and numerous Democratic congressional candidates and in opposition to Romney and numerous Republican congressional candidates.

10) FreedomWorks for America, $3.66 million, in opposition to Obama and in support of numerous Republican Senate candidates, including Rep. Connie Mack (R-Fla.), George Allen of Virginia and Josh Mandel of Ohio. Also opposing a number of Democratic congressional candidates.

Notable for not being on the list: the Koch brothers-backed Americans for Prosperity, who briefly — and aggressively — delved into expressed advocacy against Obama but decided to return to producing issue advertisements after a federal court last month ruled that groups doing so wouldn’t have to reveal their donors.

In all, two dozen outside political groups — political action committees, super PACs and politically active non-profit groups — spent more than $1 million between Sept. 15 and Sept. 30 to advocate for or against federal candidates, federal records indicate.

On the right, American Future Fund, the Ending Spending Action Fund and YG Action Fund/Network are among those to each spent between $1 million and $3 million for late September.

On the left, the League of Conservation Voters, Majority PAC and Planned Parenthood political committees spent into the low seven figures.

The amounts don’t include money spent by groups on issue-based advertisements, which may mention candidates but don’t overtly call for their election or defeat.