I'm looking through old election returns and saw that a candidate in the 1942 Michigan Senate election is listed as a "sticker candidate".
What is a "sticker candidate"?
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I think it's another name for a write-in candidate. According to Wikipedia's page on write-in candidates (under the heading "How to write in the name"):
Some U.S. states and local jurisdictions allow a voter to affix a sticker, with the write-in candidate's name, to the ballot in lieu of actually writing in the candidate's name.
Wikipedia's page on the 1942 Senate elections, under the heading for the state of Michigan list Gerald L. K. Smits (the one from your picture) with the label (sticker), just like in your picture.
On Wikipedia's page on the 1942 Senate election in Michigan he has the label (write-in) instead.
As such, I'd conclude that (sticker) indicates he was a write-in candidate.