Sen. John Cornyn said a tightening presidential race has helped incumbents.
Rubio faces a tough reelection battle against Rep. Patrick Murphy.
Late last month, Rubio challenged Murphy to six debates.
The Senate Leadership Fund will focus on convincing swing voters to get behind Pat Toomey.
“It’s a very, very competitive environment,” the Senate leader told The Hill.
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.