His argument is that if the popular vote had been what the candidates were competing to win, he would have run a different kind of campaign and won anyway, won an even bigger victory. This was George W. Bush's argument back in 2000. And there's significant merit to it. The whole campaign would have been run differently on both sides.
You can be sure New Hampshire would have received essentially no candidate visits, while New York, California, Texas, Illinois and other states would have gotten many, many more.
But really Trump is wrong. Lot of good it does them, of course. But Democrats in our current political alignment, would be inherently advantaged in a popular vote system. The logic is simple. Democrats have more big states with lopsided victories where votes are 'wasted' in electoral terms. Sure, Donald Trump would campaign more in New York and California and Texas. So would Hillary Clinton. But overall this would leave Democrats with more votes to run up.
After all, it's no accident that the presidential winner has lost the popular vote only four times in our history and two of those four times were in the last sixteen years. Sticklers will note that it happened a fifth time in 1824. But there was no developed two party system at the time. So it's not a comparable example. Also notable, one of those other four times, the 1876 presidential election, was bound in the end of Reconstruction and a battle for who controlled key Southern states. In other words, there's a decent argument that in about 175 years of a mature two party system, this disjuncture between electoral college and popular vote has only happened three times - and twice in the last 16 years.
That is a problem. Yes, it's a bigger problem for the Democrats now. But I think when people step back from the intensity of the moment and look at the big picture they will see it's not a good thing for the country either.
For present purposes, yes, both sides would run different campaigns. But it would open up many more opportunities for Democrats than Republicans.