Johnson called Rounds a friend and welcomed him to the race. He also praised Rounds for refusing to sign the "no tax pledge" offered by Grover Norquist and Americans for Tax Reform.
"I had an excellent working relationship with him during his eight years as Governor, and the fact that he has already re-stated his refusal to take the Grover Norquist 'no tax increases under any circumstances' pledge is a very good sign," he said.
"The fact that Mike realizes that it is irresponsible to take tax increases off the negotiating table is absolutely to his credit," he added.
Johnson has served in the Senate since being elected in 1996, and headed the banking panel following Sen. Chris Dodd's (D-Conn.) retirement in 2011.
Johnson was sidelined for nearly a year after he suffered bleeding in the brain in 2006, and has largely been confined to a wheelchair since then.