Heller was the only one of the eight Republicans up for reelection in 2018 that is 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 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.

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It's not just that Democrats have so many vulnerabilities. It's that Republicans have so few.

Heller's decision doesn't take the Nevada seat off anyone's radar. He only won the seat with 46 percent in 2012. And Democrats have to feel emboldened in the state after winning the open seat of now-retired Nevada senator Harry Reid (D) in 2016. But Heller represents, by far, Republicans' best chance of holding the seat in fall 2018. And having to beat an incumbent rather than run for an open seat may scare away some of the stronger Democratic candidates.

In Arizona, Republican Sen. Jeff Flake's biggest problem at the moment may well be in a GOP primary — given that he was an outspoken voice against Trump during the presidential race. Democrats were extremely optimistic about their chances of knocking off Sen. John McCain (R) last November, but even with a solid candidate in former representative Ann Kirkpatrick (D), the race wasn't terribly close. In a midterm election — and the expected whiter and older electorate — it's hard to imagine Flake as deeply endangered unless he does something colossally stupid (always a possibility!) between now and November 2018.

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Beyond those two seats, there are simply no opportunities for Democrats. Like, none.

What that means is that the much-coveted-almost-never-attained filibuster-proof Senate majority — 60 seats — is possible, if far from probable, for Republicans in 2018. To win, Republicans would have to come close to running the table in the 10 states that Trump won — no easy task. In Michigan, Pennsylvania and Wisconsin, Trump's victory margins were tiny and the Democratic incumbents are not likely to get caught off guard. Ditto Florida and Ohio, which Trump won by slightly larger margins.