But the long-term trends are nationalization (voters perceive their representatives through the lens of national and presidential politics) and polarization (voters see the parties as distinct and agree more with one side). Voters learn less about their own legislators and more about the president, in part due to decreasing reliance on local news. As a result, fewer voters split their tickets, voting for one party’s candidate for president and the other’s for Senate or the House.
Democrats have faced the same problem in trying to distinguish themselves from their party. Voters recognized the independent streak of West Virginia’s Joe Manchin and Montana’s Jon Tester in the 2018 midterms, but Missouri’s Claire McCaskill, North Dakota’s Heidi Heitkamp and Indiana’s Joe Donnelly weren’t able to overcome the Republican lean of their states. Manchin went so far as to appear in ads showing him shooting at policies he disliked and proclaiming “for me, it’s all about West Virginia.” He won a state that Hillary Clinton lost by more than 42 points.
Nationalization makes it more difficult for senators to be seen as separated from their party’s president and his priorities. So even if Republican senators do break with Trump, fewer voters now learn about it because they no longer see state-specific news. Since voters tend to assume that partisans vote like their parties, voters are often unable to perceive moderate senators’ divergent policy positions.