I found this poll interesting. It's the newest Rasmussen Poll -- done on November 9th and 12th of a random national sample of 1,000 likely voters, with a 3 point margin of error.
It shows that 59% (nearly 6 out of 10 people) think it's likely that the Democrats will regain control of Congress in the 2018 election. Only 34% think the Republicans will retain control, and another 7% are unsure.
It's still a year until that election, and the real results will depend on turnout. Voter turnout is traditionally low in off-year elections, but there is a lot of enthusiasm among Democrats right now. If they can get large numbers of voters to the polls next November, then they can win back control of Congress -- if they can't do that, then the Republicans will retain control.