Right now the Supreme Court hangs in the balance. The President has the ability to nominate someone to the court, and the Senate has to approve. Currently we have a Democratic President and a Republican held Senate. This guarantees that someone left leaning will be appointed, but they have to be palpable to the Republicans as well. If both the Senate and the Presidency were of the same party then the next Supreme Court justice will be either further right or further left. With a four seat advantage and 34 Senate seats up for grabs in the fall, the GOP has a very good chance of losing the Senate and the Presidential election due to the craziness of Trump. The NRA with a liberal sided court will lose its ability to impact Second Amendment legislation.
Because of the down ballot effect that Trump presents, it may be in the best interest of the NRA to take a back seat and focus their influence on the closer Senate races to at the very least ensure a more centrist Supreme Court nominee.