Open Secrets:

While much of the focus this election season has been on super PACs — which can take money from almost any source, but must report their donors to the FEC on a regular basis — these other outside groups have escaped the same scrutiny. That’s in part because we know much less about them, other than the limited spending totals they have to report. Most are 501(c)(4) “social welfare” organizations under the tax code, and their donors can present them with $5 million, $50 million — any amount at all — without fear of having their names made public. Even the IRS will see only group’s topmost donors.

Here are the 10 candidates who have drawn the most shadow money spending:

1) Barack Obama (D)

Support: $1.1 million

Oppose: $74.1 million

2) Mitt Romney (R)

Support: $8.8 million

Oppose: $5.1 million

3) Tim Kaine (D-Va.)

Support: $11,000

Oppose: $11.4 million

4) Sherrod Brown (D-Ohio)

Support: $100,000

Oppose: $9.7 million

5) Tammy Baldwin (D-Wis.)

Support: $78,000

Oppose:  $7.9 million

6) Shelley Berkley (D-Nev.)

Support: $39,000

Oppose: $6 million

7) Jon Tester (D-Mont.)

Support: $1.1 million

Oppose: $4.7 million

8) Bill Nelson (D-Fla.)

Support: $0

Oppose: $4.2 million

9) Dean Heller (R-Nev.)

Support: $572,000 million

Oppose: $3.5 million

10) Joe Donnelly (D-Ind.)

Support: $0

Oppose: $3.3 million