Sen. Johnson returned to the Senate floor six weeks ago after a nine-month absence to recover from a nearly fatal brain hemorrhage. In a letter to supporters, Johnson says he's been back on the job long enough to know that he can do the work.
During a recent visit to Sioux Falls, Johnson told reporters he wasn't sure about his plans.
"I've always announced formally that I will run for re-election in January or February," Johnson said then. "This time I face unique circumstances."
And that's all Johnson said about it, then. Johnson's new letter to supporters says he's always planned on running but he wasn't sure if he could handle the job. He says he's confident that what he calls his second chance at life, lets him focus on advocating for the people of South Dakota.
There will likely be challenges in the campaign. Last week, Johnson told reporters he is frustrated with his recovery.
"The frustrating efforts go to my concern that my intellect is not reflected in my speech," the senator said.
Johnson says his mind works much faster that his speech.
Johnson's campaign reports having more than $2 million on hand. Several key Republicans have waited to see what Johnson was doing before throwing their hat in the ring.