They're worried

With good reason.

The president, who had been warned aboard Air Force One that the crowds at the arena [in Tulsa] were smaller than expected, was stunned, and he yelled at aides backstage while looking at the endless rows of empty blue seats in the upper bowl of the stadium, according to four people familiar with what took place.

[...]

Outside advisers to the president said his team was fielding calls from nervous donors and Republican lawmakers, who were asking whether the poorly attended rally indicated problems that were too big to fix with just over four months until Election Day.

  NY