On the one hand, Trump doesn't win if McConnell allows Garland's nomination to go through. The religious right was persuaded to embrace Trump because the Supreme Court was on the line. If there wasn't an open seat, in particular one replacing Scalia, then he probably loses a few percent of the vote from that crowd and Clinton wins.

On the other hand, Obama and the Democrats were also gambling that they could get a more liberal justice than Garland through if Clinton won. There were ways for Obama to force McConnell's hand or make the GOP pay a much steeper price for blocking Garland. The Democrats could have ground the Senate to an absolute halt by denying cloture and unanimous consent on every piece of legislation. They still had to pass a budget that year to keep the government funded. In retrospect, not doing that was a huge miscalculation.