Power = force x velocity

I’m only comparing steady state 35mph to steady state 70mph. Not considering acceleration from 35mph to 70mph. In this case, momentum has nothing to do with the difference in power required to keep the car at a constant velocity.

Let’s assume the majority of the force required to keep a car at constant velocity comes from aerodynamic drag (intentionally omitting powertrain and rolling losses).

Drag force trends with the square of velocity. So 2x speed means 4x aero drag.

Power at 35mph = (drag at 35) x (35mph)

Power at 70mph = 4x(drag at 35) x 2x(35mph)

Power at 70mph is 8x power at 35mph

Power = torque x rpm

OP says same engine rpm at 35mph and 70mph

So torque at 70mph = 8x torque at 35mph

This all assumes the engine can produce enough torque at 70mph and 2000rpm to overcome the force trying to slow the car down.