WAUSAU, Wis. — The presidential candidates rarely trek this far north, but on election night, experts in the Wisconsin vote said, they would pay close attention to the vast region west and north of Green Bay, viewing it as a telltale of whether the state’s political upheaval since 2010 has undermined President Obama’s bulwark of support.

The rural northern tier may be the most volatile area of the state, where Mr. Obama flipped 19 counties from red to blue in 2008, all of which then shifted back, hard, in a Republican wave two years ago.

On Wednesday, Barb Amsrud cast an early vote at City Hall here for Mitt Romney, a switch from her 2008 choice of Mr. Obama. “Honestly, I’m not totally crazy about Romney, but we have to do something because what we have now is not working,” said Ms. Amsrud, 58, who works in health care.

Moments later another early voter confided she switched to Mr. Obama after voting four years ago for Senator John McCain, his Republican rival.