With President Trump's base support beginning to erode and a Republican-held Congress failing to achieve any of its chief policy aims, the stage seems set for a Democratic takeover of the legislative branch in 2018. But Democrats are staring down one huge obstacle: the Senate map.

Expand chart Data: Dave Leip's Election Atlas, U.S. Senate; Chart: Andrew Witherspoon / Axios

The problem: 25 Senate seats currently held by Democrats or independents caucusing with them are up for election in 2018 — more than half of the Democratic caucus. Even more significantly, 10 of those seats are in states that voted for Trump in 2016. Conversely, just eight Republican seats are up for election with only one in a state that went for Hillary Clinton.

Think of it this way: 538 presented a scenario that should terrify Democrats: it took Iraq, Katrina, the start of the Great Recession and rock bottom approval ratings for George W Bush for the Dems to grab a 60-seat Senate supermajority in 2008. But 30 states went for Trump in 2016, meaning the GOP would just need to nab both senators from those states in order to attain a supermajority — not unthinkable in today's hyperpartisan, straight-ticket environment.

It's not over — yet: A RealClearPolitics analysis showed that — while 2018 is certainly bad for Democrats — next year's map isn't historically bad. Based on trends, Democrats are likely to lose a few seats, but that might put them in a better spot with a more favorable map and a presidential race in 2020. It's also worth noting that Democrats managed to pick up two seats, in Missouri and Indiana, with the same map in 2012.

Three things in Democrats' favor: