The betting sites react very quickly to new information, so they provide an excellent way to monitor events in real time.
Granted, in 2012 my favorite market (InTrade – RIP) was clearly manipulated. Had anyone been set up to perform the arbitrage, they could have pocketed a few million risk-free dollars on election day. My guess is that the arbitrageurs are ready this time around, which means it does not matter which betting site you watch; any attempt to push on one requires pushing on them all.
That said, my favorite these days is Betfair, and all of these links go there. It is a fairly liquid market. As I write (Nov 7 12:30 EST), they have $137 million in matched bets on the overall election outcome.
Click the “Back & Lay” check box to see the ask/bid spread. You can think of the number as a reciprocal probability (e.g. back/lay of 2 means probability of 1/2 = 50%). Or you can think of it as the number of sides in a die roll (e.g. 6 = 1d6, 2 = coin toss, etc.)