As President Donald Trump continues to struggle, Democrats have moved into a stronger position to challenge Republicans for control of Congress next year, according to a new NBC News/Wall Street Journal poll.

The survey shows that just 40 percent of Americans approve of Trump's job performance, while 55 percent disapprove. That's virtually unchanged since last month.

At the same time, Americans now say by 50 percent to 42 percent they want Democrats to control Congress after the 2018 mid-term elections. That's the largest lead either party has held on that generic ballot question in the NBC/WSJ poll since 2013, and the first time either party reached 50 percent on that question since 2008.

Such "generic ballot" questions provide only a rough guide to outcomes in Congressional elections, since results vary widely with the partisan composition of individual districts. Republicans have won all four of this year's special House elections to replace Republicans chosen by Trump for his Cabinet, including contests this week in Georgia and South Carolina.

But the stronger Democratic position reflected in the NBC/WSJ poll mirrors the fact that Democratic candidates in those special elections ran more strongly than Democrats in the same districts in 2014 and 2016.