Rauner doesn’t excite his base. From a Republican point of view, he failed on every count. He made Illinois a sanctuary state. He did nothing about the budget. He’s been completely knee-capped by Madigan, the state tax is back up. He’s been flailing for over a year.
The only way he wins is if Dem voters simply don’t show up for Pritzker. Which is possible because it’s hard to get excited about Pritzker. In short, it’s the Dems’ election to lose. But they tend to be very good at losing unlosable elections and winning the unwinnable ones.