If you mean in congress that would be in 1999 when Micheal Forbes of NY switched to the RATS. We got the last laugh however since he was defeated in the 2000 dim primary and we won he seat back the same year.
The Republican Party is often called the "stupid party", but the ones who hold that seat are exceptionally stupid. In 2002 we lost the seat again because freshman Grucci made a wild claim that his opponent was ignoring rape allegations on his campus. This turned a near certain win for Grucci into an upset loss.