Trump has moulded the Republican Party into his own organism. His base, with its voting power, keeps Republicans in Congress mostly compliant. Now the party's legislative control goes on the line. The outcome will have an impact on what happens in the rest of his term and his re-election chances.
The Democrats are considered to only have an outside tilt at regaining Senate control, but have a stronger chance of taking the House. Polls note a voter enthusiasm gap in their party's favour and it also leads in the RealClearPolitics.com generic ballot by 7.8 per cent.
History suggests voters push against leaving the three branches of government in one party's hands. The University of Virginia's forecast says: "The President's party has lost ground in 36 of 39 House Midterms since the civil war with an average loss of 33 seats ...
Since the end of World War II, the average seat loss is 26 seats, or right on the borderline of the 23 net seats the Democrats need to elect a House majority." It has been hard to get a real read on Trump's standing.
Good economic conditions, including low unemployment, and his incumbency are in his favour. How much does the Russia investigation count outside Washington and Twitter?
There's also the support of his base and right-wing media, co-operation between the presidency and Republicans in Congress.
Since he blitzed the Republican primary field in 2016, Trump has resisted trying to grow support, happy to play to the converted. Trump is relying on the intensity of that support.
This has meant that his popularity in polls tends to have a ceiling of 45 per cent.
Most Americans are unmoving in their disapproval of him. Of key importance is the growing group of Independents.
An NBC/WSJ poll this week had 58 per cent disapproving.
A major difference between 2016 and now is the Democrats have been jolted out of complacency and somewhat rejuvenated.
This has been shown through special election wins, aggressive fundraising, a flood of female candidates and novices standing.
They have someone to oppose and measure their values against.
Yet, the Democratic top leadership bench is old and the younger crowd lack name recognition. And does anyone know how to take on Trump, one-on-one? The way it's all about him, whatever it is, is a big hurdle for the Democrats.
Trump is awash with problems. But the wall of noise he brings prevents other messages getting through effectively.
