The Wisconsin recall election for GOP Gov. Scott Walker has become the most expensive contest in state history, a non-profit watchdog group said Sunday.

The Center for Public Integrity (CPI) said more than $63.5 million had been spent by Walker, his Democratic challenger Milwaukee Mayor Tom Barrett and outside groups ahead of Tuesday’s vote. The tally which includes funds spent since November 2011 topped the 2010 gubernatorial contest, which held the previous record of $37.4 million.