The good news for Senate Democrats is that, given the party’s healthy lead in generic-ballot polling, they are likely to overperform again this year.6 The bad news is that overperforming by enough to take back control would be unprecedented (at least in modern history). As mentioned above, Democrats’ -9 net FRITZ score in 2018 represents their worst position since 1992, and their best overperformance during that time frame has been +7. If Democrats repeat this and win seven more seats this year than partisan lean would suggest,7 that would translate to a Republican gain of two seats. Democrats need to overperform by a whopping 11 seats in order to snag a majority.

Still, Democrats should probably be thrilled with an overperformance of even half that. It all comes back to that pesky Republican bias in the Senate — and specifically its lopsided distribution. In short, 2018 could be not just bad, but a veritable armaggeddon for Senate Democrats. They should count their lucky stars that their worst-case map looks like it’s going to coincide with their best-case turnout environment.

The Senate isn’t just biased toward Republicans; it’s really biased toward Republicans. Going by partisan lean, there are 31 states more Republican-leaning than the nation as a whole compared with just 19 states more Democratic-leaning.