Historically speaking, presidents don't tend to hold onto power in congress for long.
But 2018 is an extremely atypical year.
The House is, of course, gerrymandered all to hell. It would take a ton of voter anger at Republicans to flip that body to the Democrats.
As for the Senate, the Democrats have made gains in three straight Class 1 elections: 2000, when Gore won the popular vote, 2006, when Dean's fifty-state strategy paid off with House and Senate majorities, and 2012, when Obama was reelected. They're defending more than twice as many seats as the Republicans, ten of which are in states Trump carried. It's difficult for me to see how they hold onto all of them, especially seats like Heitkamp's in North Dakota or Manchin's in West Virginia.
Plus the DNC just pissed off a good chunk of progressives by electing Tom Perez as DNC chair--the one candidate for that position who had no endorsements from Sanders supporters. Every other candidate had endorsements from both Clinton supporters and Sanders supporters, while Perez had only been endorsed by people on the Clinton side. How this anger manifests itself--maybe through primary challenges to people like Heitkamp and Manchin, or through a boycott at the November polls--remains to be seen.