Even if we massively downgraded the US military, reduced the air force, navy, army, marines and put them exactly on par with the Russians in terms of technology, manpower and equipment , took away all allies, we removed the vast gun ownership by the general population. The US still has one massively unassailable advantage.

Geography.

The nearest major naval base to the US is about 5000 miles. By sea, thats more than a week's worth of travel going full steam. Which realistically is the only way to bring the absolutely massive amounts of troops and supplies needed to invade the US, given that most of Russia's transport aircraft can't even fly that distance when loaded.

So after a week of travelling by sea, and having given plenty of time for the US to mobilize its forces and prepare a defense, you now have to somehow make a contested landing with what's left of the navy that didn't already get sunk on the way over. What's left of that landing now has to fight in heavily built up areas, of which either coast is going to be a nightmare but LA would be the worse of the two given the large open spaces where snipers positioned in tall buildings could pin down entire companies and call in air and artillery strikes as needed.

New York is going to be no fun either because you essentially have to demolish each building you come across like what we see the Russian army doing to various fronts in Ukraine, the only way they've been able to make gains is to remove ALL defensible positions otherwise your army gets picked off and ambushed as they make their way through the cities.

While all this is being done, your troops need ammunition, fuel, food which is 5000 miles away while the US has everything it needs in its backyard. This invasion is dead in its tracks before they hit central park in NY or Disneyland in LA.