A small-town Mexican mayor was shot to death Wednesday while campaigning for President Felipe Calderon's sister in a governor's race for the state of Michoacan, local media and party officials said.

Officials with the National Action Party (PAN) of which Calderon and his sister, Maria Luisa Calderon, are members said the mayor of La Piedad, Ricardo Guzman, was gunned down.

Calderon expressed condolences via his Twitter account, calling Guzman an honest man who wanted a better Michoacan.

Local media said Guzman was leafleting for his party ahead of the November 13 election when he was killed.

We will continue to campaign, Maria Luisa Calderon told local television.

Michoacan is where Felipe Calderon began his political career and is a violent flashpoint of the campaign against drug cartels that has left more than 45,000 dead nationwide since Calderon took office in December 2006.

(Reporting by Anahi Rama; editing by Todd Eastham)