Democrats are questioning if a $10 million payment broke federal law.
The progressive icons are hitting the stump for candidates in key Senate races.
Rep. Todd Young (R-Ind.) faces a tough race against Bayh.
In all four states, the incumbents are polling better than Donald Trump.
The move comes after outside groups also pulled ads backing Strickland against Rob Portman.
Timothy Cama 09/09/16 10:00 AM EDT
An environmental group and steelworkers union are putting nearly $4 million into a super-PAC.
Democrat Katie McGinty is looking to unseat GOP Sen. Pat Toomey.
The ads support Democrat Deborah Ross as she seeks to unseat GOP Sen. Richard Burr.
Democrat Ann Kirkpatrick is trying to unseat McCain in a reliably red state.
Cornyn said he won't get involved in a primary fight next cycle.