Lexington Democratic Mayor Jim Gray has won the party's nomination for Senate.
He has an unfavorable rating among Republican voters and the general electorate.
The super-PAC's video targets Portman's support for Trump.
The network is making its most aggressive intervention yet into the 2016 cycle.
The pitch comes after a confrontation over ethics violation allegations against Grayson.
Locked in a tight race in Illinois, the two have released ads in the last two days.
Johnson accused Democrats of trying to "politicize" his comments.
"He wants us to be just another country," Carlos Beruff said.
Sen. Ron Johnson used a 9/11 story to highlight the importance of voting.
"Why do you perpetuate these lies this way?" Grayson asked.