No way do they have anything close to 2008. In 2016 Hillary got 2% fewer votes than Obama did in 2012. Obama was an exceptional candidate, and I mean that in several ways. There was no way Hillary could expect every Obama voter to vote for her, and it is even less likely they will get 2016 type turnout for the midterms, let alone anything resembling 2008. They have nothing to vote for, only fear and hatred of Trump, and that isn’t a motivating force beyond the base. In addition they have no galvanizing candidate to unite them.
Just my opinion of course. On the other hand, Trump has given every reason and worked very hard to get out the GOP vote. And the Dems have given the GOP many reasons to want to stop them. I think the GOP holds the house and expands the Senate by 4-5 seats.
In 2016 we only had maybe one or two clipboard wielding people going door to door for Hillary, and you had to chase them down to figure out who they were working for. This is more like the primary battle between Obama and Clinton which got the ball rolling for the turnout in the general. That points to a massive GOTV effort which is not the same thing as a wave election. A wave election is an organic movement of people who are motivated to vote for or against someone and who get themselves to the polls. This is not that. This is a massive ground game. This is paid and unpaid people going door to door.
The RNC had better be on its game in GOP areas because the Dems are not messing around here.