Sen. Ron Johnson (R., Wis.) endorsed Reince Priebus this morning via a letter to RNC members:

Dear RNC Members,
It’s difficult to argue with success. The political turn around that occurred in Wisconsin on November 2nd was a welcome event for us all. And Reince Priebus worked hard throughout the campaign to help make it possible.
His efforts were particularly helpful to me. With no previous political experience, I had to have a trusted confidant to rely upon and a strong state party organization that enabled my campaign to reach deeply into the grassroots. Reince provided both.
As Chairman of the Republican Party of Wisconsin, Reince assembled a highly qualified staff and put together one of the most aggressive and successful grassroots organizations in the nation. This operation delivered an extremely effective volunteer voter contact effort that helped elect Republicans all over the state. This is the kind of demonstrated leadership and commitment to success that we need at the Republican National Committee.
Reince worked tirelessly, making himself available to me day and night. I am confident that Reince brings the experience, commitment and work ethic the Republican National Party needs to achieve electoral success in 2012. I encourage you to support Reince for Chairman of the Party.
Sincerely,
Senator Ron Johnson
State of Wisconsin