If Ds had won Bill Nelson's seat in 2018, how much easier would it be to get a majority?

[Roll Roons]

Helps significantly. Assuming Democrats write off Alabama as a loss and build solid leads in Colorado and Arizona as they are doing now, they would only need two more seats to get to 51. Between North Carolina, Maine, Montana (assuming Bullock still runs), both Georgia seats and Iowa, not to mention reach targets like Alaska, Kansas, South Carolina, and Texas, it would be a very doable task and I think they would be favored, though not guaranteed, to flip the Senate.

In real life, it's still very possible, especially with Trump crashing and burning, but it's a tougher task than it would have been if Nelson had held on. Though it's not just Nelson - there have also been other close races in recent cycles that could have gone the Democrats' way, but didn't. Think Pennsylvania in 2016, or Colorado and North Carolina in 2014. Mark Udall's seat would have been Likely to Safe D, and Kay Hagan, assuming she doesn't die, would have also likely been favored.