Joan Ruff (search), an attorney and business executive, hopes to become the first Democrat to win a Senate election in Kansas since 1932.
Ruff, 56, describes herself as a political moderate. She said she will make health care and education major issues.
Sebelius issued a statement supporting Ruff, the first Democrat in the race. "I believe she will work hard on behalf of the people of Kansas and look forward to supporting her candidacy," Sebelius said.
Brownback served two years in the House before he was elected in 1996 to finish Bob Dole's Senate term, then was elected to a full six-year term in 1998.
A spokesman, Aaron Groote, said Brownback "is focused on running for re-election by doing a good job and working hard for the people of Kansas."