Which of the following is a current U.S. Senator from Wisconsin?

Ron Johnson is a current U.S. Senator from Wisconsin, having first been elected to the Senate in 2010. He represents the state in the Senate and serves on various committees, focusing on issues that affect Wisconsin and the United States as a whole. Johnson's position is a significant one, as U.S. Senators play essential roles in shaping federal legislation, representing their constituents, and influencing national policies.

The other individuals listed are notable politicians but not current U.S. Senators from Wisconsin. Barack Obama served as a U.S. Senator from Illinois before becoming the President of the United States. George W. Bush was the President and previously served as the Governor of Texas, but he has never been a U.S. Senator. Pete Buttigieg is currently the Secretary of Transportation and was previously the mayor of South Bend, Indiana, not a U.S. Senator.