The Wisconsin race is already expected to be a highly nationalized one, with Democrats battling to keep the seat in the midst of fallout from this year's epic fight between state workers and Gov. Scott Walker (R). Ryan's entry into the contest would add another level of national focus, as Ryan's plan to end Medicare as we know it, as part of the House budget plan he wrote, has become a national rallying cry for Democrats.
For their part, Democrats say they've got a full field of potential candidates at their disposal, including former Sen. Russ Feingold and Rep. Tammy Baldwin.
Read Ryan's full statement:
"In over two decades of service in the Senate, Herb Kohl has done much to help the great state of Wisconsin and the lives of its residents. While Senator Kohl and I have had our policy disagreements in the past, he has always had my respect. It has been a privilege to work with him over the years, and I wish him the best in his future endeavors. I was surprised by Senator Kohl's announcement and want to take some time over the next few days to discuss this news with my family and supporters before making any decision about how I'm best able to serve my employers in the First Congressional District, our state and nation."