Curran is up for re-election in 2010.
Just three months ago, he endorsed a Republican candidate for Lake County state's attorney and he supported Jim Ryan for governor is 2002.
Curran says corruption within his former party made him turn to the GOP.
"I joined it on a matter of conscience. The hope in Illinois is in the Republican party," said Mark Curran (R), Lake County Sheriff. "Corruption, the Chicago Machine, it existed long before I was born, it'll continue long after I am gone."
Prior to becoming sheriff, Curran worked as a criminal defense attorney.