Mars' gravity is strong enough that manned missions may not be able to lift off from the surface for the very long voyage home.
A quick spin around for some hard data shows that gravity is 1.62 m/s2 on the moon, and 3.69 m/s2 -- over twice as strong. Escape velocity is also over twice as great on Mars: 5.03 km/s vs. 2.38 km/s.
The problem of course is the cost of fuel, schlepping all of it to Mars, and all of the associated complications of sending all that additional weight to Mars and to the surface.
This is not an insurmountable problem, but a successful mission profile presupposes some major advances in propulsion systems. All in the name of some empty campaign promise intended to inspire the country and gain votes.