This chart uses information from the latest Economist / YouGov Poll -- done between January 28th and 30th of a random national sample of 1,500 adults (including 1,264 registered voters). The margin of error for all adults is 3 points, and for registered voters is 2.9 points.
It shows that Democrats still retain a 5 point advantage in the general public (and among registered voters).
When you break it down demographically, Republicans have a slim advantage among Men, Whites, and those 65 and older. But those advantages are countered by bigger margins for Democrats among Women, other races/ethnicities, and other age groups.
The 2018 election is a golden opportunity to flip Congress (at least one house of it). But that will only happen if the Democrats actually turn up at the polls in November.