SPRINGFIELD — Plaintiffs arguing against a sweeping pension reform law are trying to undercut the government’s main defense for the changes. Riding the momentum of a favorable Illinois Supreme Court decision last month in Kanerva v. Weems, the group filed a motion Thursday in Sangamon County Circuit Court that says the state can’t use “reserved sovereign powers” as a defense of the new law because the Illinois Constitution is ironclad in its protection of pension benefits. In a 14-page …