- Watch Robert's explainer about why the mid-terms matter and what's at stake
This week we will know the shape of the new US Congress. The Senate is likely to stay in Republican hands, but we will soon discover whether the Democrats have seized control of the House of Representatives.
If they succeed, and Republicans suffer a set-back, then Donald Trump will face a truly miserable two years.
His hopes of new legislation to further his agenda will be over. Crucially, Democrats will gain control of all the House committees. And on Day One the investigations will begin. They will demand to see Trump’s tax returns; they will open aggressive probes into his environmental and immigration policies; into his financial conflicts of interest; and of course into his attempts to silence those women who have accused him of sexual harassment.
If they gain power, the House Democrats’ aim is to be the Boa Constrictor of American politics - the slow strangulation of what they regard as a dangerous, divisive and rogue presidency.
So Donald Trump may not be on the ballot on Tuesday (these mid-term elections are for Congress, not for the White House), but he has a huge stake in the outcome. Indeed, in many ways this is a referendum on his opening two years.
In Pittsburgh, I saw people determined to vote. One man told me that if necessary he will crawl to the ballot box. They have been further motivated by the tragedy of Saturday's massacre. Many people - including one man who was in the synagogue and hid in a storeroom - believe that Trump’s inflammatory language has empowered white supremacists and paved the way for anti-semitic attacks.
The genius of Trump is his ability to infuriate the political left and provoke an angry reaction, thereby making his core supporters even more motivated and combative. It’s a vicious cycle that has made American politics a blood sport.
It’s not pretty. Many groups and communities feel vulnerable and violated - whether its LGBT individuals, Jewish Americans, immigrants, Hispanics, Native Americans - and they will vote on Tuesday in significant numbers. Anger is a powerful mobilizer.
But against them are pitched those in Middle America who feel the economy is improving, who love a President prepared to defy convention, and who loathe political correctness.
This week will tell us much about America’s immediate political future. It will also give us an insight into the country’s political soul. Will they give an inflammatory President a second chance or will they slam on the brakes?