Tommy Thompson was a popular four-term Republican governor of Wisconsin from 1987 to 2001. He then served from 2001 to 2005 as the Secretary of the United States Department of Health and Human Services in the George W. Bush Administration. He also served as the chairman of the Republican platform committee in 2000 and kept the platform conservative.
He sought the Republican nomination in the 2008 Presidential Election. Wisconsin is one of just a few states that is "in play" and can be won by the Democratic or Republican nominee. Thompson was attractive to Republicans because he would likely have won his home state of Wisconsin, and that would have enabled the Republicans to capture the White House without winning Ohio, which has become more Democratic since the Republicans carried that state in the United States presidential election of 2004.
After Giuliani had withdrawn and endorsed Senator McCain, Tommy Thompson also endorsed him. Thompson said, "Today, I am proud to announce my endorsement of John McCain for president, John McCain has a lifetime of service to our country, and he is the leader best prepared to serve as commander in chief from day one. His conservative record and personal leadership qualities give us the best chance of defeating the Democrats in November." 
On April 15, 2009, Thompson announced he would not be running for US Senate against Russ Feingold, ending speculation that he would run. Thompson was considered by many to be the most likely to defeat Feingold and win the seat for the Republicans.