WASHINGTON — Rising tensions between the US and China are widely considered the top threat to America’s national security, but the issue got very little airtime during the first Republican presidential debate last month. Ahead of Wednesday night’s second face-off, Rep. Mike Gallagher (R-Wis.) told The Post he hopes to see the potential 2024 GOP nominees “make a pitch to the American people about how and why we must win the ‘New Cold War’ with China.” “I think the candidates, to pass the commander-in-chief test, — given that it’s at the Reagan [Presidential] Library — just need to communicate in…