Kirk, however, is expected to win the GOP nomination.
Until Tuesday, Johnson was one of two GOP Members in the delegation holding out on endorsing Kirk; Rep. Don Manzullo (R-Ill.) has said that has a policy not to pick sides in primaries.
“Mark Kirk shares my commitment to job creation, economic growth and care for our nation’s veterans,” Johnson said in a statement. “Mark will be a Senator we can all be proud of.”
CQ Politics currently rates this general election contest as a Tossup.