He absolutely would have. Pat Quinn lost this for being Pat Quinn. A generic D (which Daley very much is) wins against Rauner.
Why was Quinn disliked so much? He has a proven progressive record. Medicare expansion, gay marriage, environemental protection, minimum wage, increased eduction funding and he even cut spending. And last but not least, he handled the situation after Blogo's impeachment pretty well.
I heard that Quinn had a difficult relationship with State Legislators (of his own party). Might that be a reason? Or the economy? California for example, another deep blue state, has economical difficulties as well, but Brown handily won.
People don't care about social issues put to rest when the economy still sucks, and Rauner was not a social issues warrior like Bill Brady. Pet social issues don't win elections; it's always about the economy.
Illinois pension crisis continued to grow from $83 billion in 2011 to nearly $100 billion now, despite Quinn raising taxes to deal with it in 2011 and gaining over $30 billion in new revenue. Quinn said it would only be to 4% and temporary, but raised it to 5% and actually campaigned on making it permanent. Also, Quinn actually cut educational and environmental spending overall, while there was bureaucratic mismanagement in the remaining funds. Quinn was stained in this past year by a number of scandals and investigations that tarnished his "clean reformer" image like the anti-crime fund that turned bad and revelations about improper payments to various Democratic party members in Cook County.
On top of this, Quinn ran a bad campaign, and Rauner ran a decent one. Rauner campaigned solely on economic issues, taxes, the pension crisis and state debt, reform, and education. Quinn primarily tried to paint Rauner as another Romney + Brady, and came off as desperate. People were angry at Quinn, and thus voted him out (but kept a heavily Democratic legislature, hilariously).
If the Illinois GOP nominated a better candidate than Brady in 2010, this would've happened then.
