Going through Mexico is very dangerous in 1.4, because the moment you have any holdings in Mexico, the US will send their DC superstack to visit. They'll have a decent supply situation by land, while you'll only have a couple of ports at the most.
What I usually do is launch a massive - multipronged assault from a couple of different locations.
1. An large amphib landing around either Boston or New York (launched from the carribean or south america)
2. A small amphib landing around Miami (launched from the carribean or south america)
3. Advance up through Mexico. (I usually put a large chunk of my armor here as that works great for wearing down the US infantry in the open desert/plains)
4. Small landing around LA or San Fransico (launched from Mexico)
5. Very Small landing around Seattle - usually just a corps of mountain troops with a single Lt Armored division (launched from LA once the port has been secured)
The biggest fighting is usually in the North east around Phily and DC and at the border with Mexico. The smaller landings in Florida and California usually don't get that much attention allowing for modest initial gains without much effort. Seattle is almost always a deserted area allowing for easy gains through the rockies
You will probably need to shuffle troops between the various fronts to respond to pressure, openinings in the US defenses.