Kirk won because it was a non-presidential ballot year. He’ll lose because this time it is.
Also had an exceptionally weak opponent as I recall.
Also elected in 2010-—a GOP year too as I recall.
Even in blue states like IL, GOP can win statewide if Dem vote turnout is low and GOP voters energized in midterm elections.
In 2010, Kirk won mainly because of the Green Party candidate, LeAlan Jones. Kirk beat the Democrat, Alexi Guinulius, by about 81,000 votes, and Jones got about 113,000 votes. If Jones hadn’t run, the majority of his voters would have voted for Alexi, helping him win.
You point to a pattern I’ve noticed in elections in recent years. Dems seem to have the advantage in presidential election years when voter turnout is much larger. In midterm elections, the electorate tends to be smaller, more disciplined, older, whiter, more conservative, and therefore more Republican than is generally the case in presidential election years.
James Carville was famous for coining the phrase, “it’s the economy, stupid.”
Now days I think it “it’s the demographics, stupid”.
And then in IL the GOP has the added difficult of having to deal with Cook County, easily the most corrupt county in the nation where cemeteries have precinct captains and ward heelers.