These two charts are from a new NBC News / Wall Street Journal Poll -- done between December 13th and 15th of a random national sample of 900 adults, with a margin of error of 3.3 points.
It shows the Democrats currently enjoy an 11 point advantage over Republicans in congressional preference for the 2018 election. That's the largest gap between the two parties since 2008 (when they had a 14 point margin).
This does not mean the Democrats will automatically flip the Congress in 2018 -- only that they will do so if the Democratic voters show up in large numbers on election day. If they don't (like happened in 2010 and 2014, then the Republicans will be victorious -- because they show up even in off-year elections). Note that Republicans had only a two point margin in 2010, but they won big -- because they voted in large numbers while too many Democrats stayed home.
Democrats have a golden opportunity in 2018 -- if they bother to vote.