Despite those dips, Heitkamp, with a net approval rating 17 points in the black, and Donnelly, with net approval of 14 points, are still among the top five Senate incumbents up for re-election this year in Trump-won states. Sen. Bill Nelson of Florida remained the most popular in that group, with a 25-point net approval.
Sens. Tammy Baldwin of Wisconsin and Claire McCaskill of Missouri, who are both seeking re-election, remain the least popular Democrats in states carried by Trump, with voters split down the middle over their job performance.
Still, midterm dynamics are in Senate Democrats' favor when it comes to the 2018 ballot: The last Senate incumbent from the party not in control of the White House who lost a midterm general election was Max Cleland of Georgia, in 2002, when then-President George W. Bush enjoyed broad support nationally. Historical data from Gallup shows Bush had an approval rating in the low to upper 60s in the month leading up to the 2002 midterm elections, roughly twice his disapproval rating.
On the other side of the aisle, Sen. Dean Heller of Nevada -- which Democratic presidential nominee Hillary Clinton won by 2.4 points in 2016 -- remains the most vulnerable Republican up for re-election. Forty-one percent of Nevadans approve of him, while 39 percent do not.
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