The bill would have required voters to show ID when voting to make it harder for people to vote multiple times, or to make it harder for unregistered voters (felons, illegal aliens, dead people, etc.) to vote. Doyle vetoed such legislation once before, saying it would be unfair to the poor. The new bill addressed his former objections. For example, it provided means for the State to provide free ID for those who couldn't afford it.
But Doyle, ever the compassionate supporter of voting rights for fraudulent voters, felt the bill would still be harmful. And he's right. It probably would have made it much harder for Doyle to get re-elected. He's going to ned ever dead, illegal, and multple-voter he and his party can dig up for the next election, because the honest voters are going to be awfully ticked off at this man.