Update: since I wrote the above the final results have come in - democrats now have a resounding majority in Virginia. Among many other spectacular victories they've taken full control of two more state-houses as well.

To understand the full significance, consider this. On Monday the GOP controlled enough state legislatures to call a constitutional convention and pass any amendment they wanted without a single democrat or independent vote. They had enough to do it all by themselves (assuming they could find one all of them agreed on).  As of today  -that power has been quite thoroughly stripped away from them. Unlikely as this scenario may be - it was possible, now it's not.