Welcome home, Mitt Romney. Not everyone is glad to see you.

The state where Romney has his deepest roots — where he was born at Harper Hospital in Detroit, met his future wife while attending high school in suburban Bloomfield Hills, saw his dad elected and re-elected governor — now looms as the unlikely challenge that threatens to upend his path to the Republican presidential nomination.

Romney trails Rick Santorum in the latest public polls in Michigan. A defeat in next week’s primary would raise serious questions about Romney’s electability and could set off a scramble for some late-starting alternative.