[Sen. Dean Heller (R-NV)] was the only one of the eight Republicans up for reelection in 2018 in a state that Hillary Clinton won in the 2016 presidential race. Of the other seven states where Republicans are expected to seek reelection, only two — Arizona (Donald Trump +4) and Texas (Trump +9) were even single digit winning margins for the president-elect. The remaining five states — Mississippi (Trump +18.6), Nebraska (Trump +26), Tennessee (Trump +26), Utah (Trump +18) and Wyoming (Trump +57.6) — are among the most Republican in the country, meaning that even if there was a surprise retirement (Orrin Hatch in Utah, at age 84, for example) it would be almost impossible for Democrats to seriously compete.
By contrast, 10 Democratic Senators are running for reelection in states Trump carried last November. Trump won half of those states — Indiana, Missouri, Montana, North Dakota and West Virginia — by double digits. That means that 20 percent of all Democratic seats up in 2018 are in states Trump won by double digits and 40 percent are in states that the president-elect carried last November.
It's not just that Democrats have so many vulnerabilities then. It's that Republicans have so few.