Across the region, more than three dozen counties in Wisconsin, Minnesota and Iowa — plus another nine nearby in Illinois — voted for Mr. Obama in 2008 and 2012 before flipping to Mr. Trump in 2016. But in Wisconsin, nearly all of those counties flipped again in 2018 and voted for the Democratic candidate for the House of Representatives, including Dunn County, where Colfax is.
There are other, more recent troubling signs for Republicans among these independent-minded voters. A survey released this past week by the Voter Study Group, which includes analysts and scholars from across the ideological spectrum, reported a 19-point drop in Mr. Trump’s favorable rating with these so-called Obama-Trump voters.
Though 66 percent of them still have a favorable view of the president, the survey noted that even slight shifts among this group, which comprised 9 percent of the electorate in 2016, could make the difference in 2020.
Whether Mr. Trump is disciplined enough to stay on message about the economy is another question. Even as the most recent jobs report bolstered his case, he diverted attention from it with a controversial phone call to President Vladimir V. Putin of Russia and then decried a ruling that overturned the result of the Kentucky Derby, attributing it to political correctness.
Many Republicans close to the White House recall the days before last year’s midterms when instead of trumpeting the economy he peddled divisive warnings about a Central American migrant caravan.
But the president’s rhetoric on issues like immigration may do as much to rally the opposition as his own base. And if turnout is higher in 2020 than in 2016, as many expect it to be, Republicans acknowledge that they will have to do more than just hold onto Mr. Trump’s committed voters.
“Simple math: Trump has to find more,” said Matt Batzel, executive director of American Majority, a group that trains conservative activists. Its work in a Wisconsin Supreme Court race last month helped elect a conservative candidate in an unexpected win that jolted Democrats.
“Unless there’s another Hillary Clinton who doesn’t campaign here,” he added, “which doesn’t seem likely to happen.”