In 2011, the Illinois House passed HB 2009, which says that no one who voted in a primary, or who filed a declaration of candidacy to run in a primary (but who then chose not to run in that primary), can then be an independent candidate in November. The law applies to all partisan office. The bill then languished in a Senate committee for almost a full year.
Illinois held its primary this year, for president and all other office, in March 20, 2012.
Then, on March 27, HB 2009, which had long been forgotten, was shifted to another Committee in the Senate. On March 29 it passed the Senate 53-3. Governor Pat Quinn signed it the next day. It has an urgency clause so took effect on March 30. So now anyone who voted in the Democratic or Republican primary on March 20 this year cannot run for any partisan office in November as an independent candidate.
This maneuver almost certainly violates due process. It is fundamentally unfair to pass a law that adversely affects primary voters after the primary is over and to make it effective so that it affects this year’s general election.
Illinois does not have registration by party. If this law had been in effect in 1980, it would apparently have prevented John B. Anderson from getting on the ballot as an independent candidate, since he voted in the March 1980 Republican presidential primary. He didn’t declare as an independent until April 24, 1980.