Illinois Republicans planned to campaign on Democrats’ alleged corruption. Then the president let Rod Blagojevich out of prison.

Looking ahead to the November general election, leading Illinois Republicans thought they had a marketable message to voters by pointing to myriad federal investigations that have ensnared Democrats at the city, suburban and state level.

But Republican President Donald Trump’s decision Tuesday to commute former Democratic Gov. Rod Blagojevich’s prison sentence on federal corruption charges may have short-circuited the GOP’s strategy.

Voters in a state with a rich history of corruption can rightfully ask Republicans how dedicated they are toward cleaning up government in Illinois when their president — who...