In the early 1970's, the USSR goes through with
Project OREL*(
http://en.wikipedia.org/wiki/Project_Orel) and by the mid-1980's, has a Navy to rival that of the US. This Navy gives them the capacity to aid the Latin-American communist movements better(albeit clandestinely), and soon, numerous Latin American communist movements gain strength. The US government loses face when it is revealed that they have been supporting horrible dictators, and is forced to cut support for right-wing regimes. Not wanting another Vietnam, Latin America falls to communism.**
By the late 1980's, the USSR faces economic crisis, and in the face of massive liberal reforms, a cabal of radical generals sieze control. In the hopes that a short victorious war would bring together the USSR, the implement
Seven Days to the River Rhine (
http://en.wikipedia.org/wiki/Seven_Days_to_the_River_Rhine). The generals do not use nuclear weapons, for fear of backlash by the public, and, obviously, retaliation. The Generals are also scared that the American "Star Wars" defense grid may have some legitimace behind it, but not enough to deter them from the invasion.
However, by the late 1980's(early 1990's?), there are sufficent forces in Europe to counter the invasion, but just barely. It becomes a stalemate, with the Soviet Navy trying unsuccessfully to counter US aid to Europe on the Atlantic and Oil shipments through the Mediterranean. However, there attempts due create an oil crisis in the United States, with all oil available going to the war effort. Attempts are made to break the stalemate, with proxy wars being fought across the globe between USSR backed nations and US backed ones. The USSR conducts air attacks and incursions into the Persian Gulf, further hurting oil supplies.***
Finally, in an attempt to break the stalemate, the USSR, with the aid of other Latin American Red states, attack the US. With the US Navy tied down in the Atlantic, the USSR is able to convoy troops to Alaska and the Pacific Northwest. Cuban and Nicaraguan forces invade Mexico, already in chaos by gurilla factions. The US, already in an oil crisis, is paralzyed by the lack of oil from Mexico and Alaska. This allows greater movement by the Soivet, Cuban, and Nicaraguan forces into the States. The US, still confident that the communists forces can be pushed back, refuses to use nukes on their own soil.
Red Dawn ensuses.
*I think there was another 1960's Soviet naval expansion program that would have better suited this scenario, but I can't find it. Soviet naval capacity is neccesary in such a scenario.
**It's doubtful that the US would turn away from it's own "backyard", but again, needed for this scenario. Maybe a big scandal could cut support for aiding Right wing regiemes.
***I can think of no other way for the US to have so little power in North America as to allow for USSR invasion than an oil crisis.