The spending shows the race is living up to expectations that it will be one of the most expensive and hardest fought in the country, as Democrats try to knock off one of their top targets in an Upper Midwestern state that was integral to President Donald Trump's victory in 2016.

Walker has repeatedly warned of a possible blue wave in November as he tries to excite Republicans who backed him three times, including in a 2012 recall election, and delivered the state to Trump. But polls show Walker and Trump's support is lagging as Democratic enthusiasm builds following a series of election victories this year.

Democrats outnumbered Republican voters by nearly 100,000 in last week's primary, where state schools chief Tony Evers emerged from a field of eight candidates to take on Walker.

But in the days since Evers' win, Walker and his supporters have dominated the airwaves. Republicans have outspent Evers and his allies since last week's primary by a margin of more than 2-to-1 as of Tuesday, based on numbers released by the groups and those on both sides tracking the ad buys.