Or maybe a lot of Americans are just tired of paying for the European defense plan aka "Operation American Meat Shield" in money and blood?
Europe wants to poke the bear and the dragon, ride the moral high horse across the globe and not pay to back any of it up. Even now the increases with a few exceptions are still rather pitiful. It's not just about 2%, it's about compensating for decades of non-investment.
And when something bad happens they collectively cower behind the US. Oh but when they do send their token military force they specifically do not want to have an American command it! No, the Americans are only good enough to pay and bleed, don't want to be associating yourself with them ...
That leads to the situation where a candidate that tells you to either pay up or go f*ck yourselves has a chance to be elected not once, but twice.