It seems Illinois used to hold it’s election for Governor in Presidential election years (Dan Walker, for example, lost the Democratic primary in 1976). Now, such elections are held in off-years (Rod Blagoevich was elected last year).
What happened to change this, and wouldn’t it have cut someone’s term short?
It did cut someone’s term short. James R. Thompson’s first term, which he won in the 1976 election, lasted only two years. Then, under a change in the state constitution, he ran again in 1978 for a four-year term.
The “good government” purpose of the change was to bring more attention to the governor’s race – to keep it from being an afterthought to the presidential races. Cynics would note that turnout is higher in years when the president is up for election, and higher turnout tends to favor Democratic candidates. Thus, Thompson, a Republican, won again in 1978 and in 1982 and in 1986. Republicans also won in 1990, 1994 and 1998.
The current governor, Rod Blagojevich, was elected in 2002, becoming the first Democrat to hold the office since the ill-fated Dan Walker mentioned in the OP.
The change was incorporated into the Illinois Constitution by the 1969-70 Constitutional Convention, which rewrote the entire document. The new Constitution was ratified by the voters on December 15, 1970. The Convention scheduled the transition, obviously, for a time that was then six years into the future.