The Department Of The Bleeding Obvious suggests to use first the current CH53 and when it is ready and flown for five years, the CH53K.
Using a radically different type of new aircraft to transport the POTUS is a sure way to kill him and to end the career of at least one Marine General.
The CH53 on the other hand, is the workhorse of several forces around the globe and is based on well-understood technology and operating principles. Despite its size it is quite maneuverable and has enough payload to be fitted with a in-air refueling probe,armour, guns, IR decoys, IR-blinding lasers (Rosboronexport has a nice one on offer) and even RADAR if you need it. But we are talking about Americans, so the easy and straightforward option won't make it...
"The MH-53J Pave Low III heavy-lift helicopter is the largest, most powerful and technologically advanced transport helicopter in the US Air Force inventory. The terrain-following and terrain-avoidance radar, forward looking infrared sensor, inertial navigation system with Global Positioning System, along with a projected map display enable the crew to follow terrain contours and avoid obstacles, making low-level penetration possible."
I especially like the photo of one CH53 carrying another one on its hook. This is a truely impressive aircraft.