Duffy was considered a front runner to take on Senator Baldwin and said she, "will be beat because her radically liberal Madison record and ideas are out of sync with Wisconsin."
The four-term Republican from northwest Wisconsin did a lot of heavy lifting for Donald Trump in the presidential race - raising speculation he was going to jump into the senate race.
"I was on track to raise a lot of money in the first quarter. Money and support was no issue for me," said Duffy.
What now? Other republicans being mentioned include Senators Scott Fitzgerald and Leah Vukmir, Representative Dale Kooyenga, former senate candidate Eric Hovde, Milwaukee area businessman Kevin Nicholson, and there's an effort to draft Sheriff David Clarke.
"It really blows the race wide open on the Republican side," said Marquette University Professor Paul Nolette.