There is virtually no crossing of party lines among likely voters in the race today. Walker is getting 96% of Republicans and 3% of Democrats; Burke is getting 95% of Democrats and 2% of Republicans. Polarization along party lines is about as extreme as it can get.
That helps Burke in one sense: there are more Democratic voters than Republican voters in Wisconsin. And it’s a big reason why the numbers change so little in this contest from month to month: massive blocs of partisan voters on both sides are locked in.