I don't really see how he would be better on national security, given that most, if not all, of our national security agencies regard Trump as a threat. He frequently gives out secrets to foreign powers (whether this is accidental or purposeful is debatable), has a distinct disregard for the military, (including doing nothing about Russia putting bounties on US soldiers, instituting a trans ban in the military against the advice of the military, calling captured soldiers losers, etc.), and he also tried to overturn an election he consistently called fraudulent in spite of no evidence found to support that conclusion and loads of evidence to conclude that the election was fair through a number of methods (fake slates of electors, organizing a mod and several senators and representatives to delay certification of the election, getting Pence to not certify).
If you meant border security, then why did he help kill a bill that would have fixed many of the things he's complaining about?
Don't Republicans support doing something now and then fixing things later? They treated fixing healthcare like that by trying to repeal the ACA without a future plan, so it seems odd that they would treat this situation different, no?