Left-wing Democrats are on a winning run - but can they lure back crucial swing voters?

On a Saturday evening in Michigan, the skies still hazy from drifting Canadian wildfire smoke, hundreds of voters packed into Detroit's opulent 104-year-old Opera House, rising from their seats to cheer loudly as Senator Bernie Sanders and Democratic Representative Alexandria Ocasio-Cortez took the stage.

As two of the leading figures on the American left, Ocasio-Cortez and Sanders were there to champion Senate hopeful Abdul El-Sayed - a doctor running on a platform of universal healthcare.