Heading into 2020, there is intense focus among campaign strategists on the weakest element of the Trump coalition: the millions of voters who disapproved of both major candidates in 2016 but took a chance on Mr. Trump. Whether an impeachment inquiry moves Obama-Trump voters like Mr. Graham off the fence, one way or the other, is a major narrative arc in the 2020 script that is rapidly unfolding and updating.

Erie County in western Pennsylvania holds a wealth of these conflicted voters. That much was clear in interviews conducted in the days after the 2016 election, and it’s clear now. Mr. Trump won an upset national victory by carrying places just like Erie County, long a blue-collar Democratic stronghold; he won here thanks to a 17-point swing from Mr. Obama’s margin of victory in 2012. The area’s flip from blue to red was a microcosm of how Mr. Trump pulled off narrow victories in this state as well as in Michigan and Wisconsin.

In interviews in Erie last month, before the impeachment inquiry began, many of the Trump voters from 2016 were either supportive of the president or unpersuaded by the Democratic alternatives. But reached by phone after the inquiry was announced, some of these voters had changed their minds. Outright conversions like Mr. Graham’s, while still rare, were reflected in an uptick of support for impeachment by independent voters in recent national polls.