Can someone explain why his early point about having lost common ground has come about? Seems likely to me that a lot of political parties might tend to come c

Can someone explain why his early point about having lost common ground has come about? Seems likely to me that a lot of political parties might tend to come closer together over time, because they will always have the support of the extremes (ie. if the Republicans move a bit closer to centre, the hard-right conservatives might grumble a bit but won't ditch them and vote Democrat) but by shifting to more centrist policies they can potentially pick up more middle-of-the-road voters. What is happening in the US that makes the reverse true, that the parties are becoming more polarised?