For clarity, then current President Gerald Ford said during a debate as part of the 1976 Presidential election campaign:

"There is no Soviet domination of Eastern Europe and there never will be under a Ford administration."

Jimmy Carter would go on to win the Presidency in that election, though probably not due to that gaffe.

For context, after WWII the Soviet Union (which was part of "The Allies") drove the invading forces of Nazi-Germany out of its territory and back through Eastern Europe until finally destroying the majority of the German military while laying siege to and then finally capturing Berlin. This was followed by the surrender of Germany and the effective end of WWII in Europe (Hitler having killed himself in the last days of the siege of Berlin when it was clear there was no route to German victory).

This left Soviet forces occupying much of Eastern Europe, which had previously been either occupied by Germany (as in the case of Poland and Czechoslovakia) or had been allies of the Axis powers (as in the case of Romania and Bulgaria). After WWII the victorious Allied Powers (the US, UK, USSR, China, et al) made agreements to divide up and take care of the administration of occupied areas. Germany was divided into zones of occupation, with the US, UK, and France each having a zone in the West and the USSR having a zone in the East. While occupied territories in the West such as France, Belgium, and the Netherlands were returned to local governance after WWII this was not true in the East. The Soviets set up communist governments in reformulated post-WWII Eastern European nation states such as Poland, Hungary, Romania, and East Germany and kept them under tight control from Moscow.

This situation led to a transition from the initial post-WWII detente between the major Western powers and the USSR into the development of the Cold War. The Soviets tightened the borders between Eastern Europe and the West and began the process of remaking Eastern European countries into Soviet client states. By the mid-1950s direct rule of Eastern Europe by Moscow had faded somewhat but been replaced by indirect rule, formalized partially by the creation of the Warsaw Pact which brought almost all of Eastern Europe into close military alliance with the Soviets. By the late-1960s there was no ambiguity about the nature of the political structure of Eastern Europe. In 1956 a revolution began in Hungary, and was brutally put down by occupying Soviet forces. In 1968 a reformist government gained power in Czechoslovakia and attempted to decentralize the economy and increase individual freedoms. The result was an invasion of the country by Soviet and Warsaw Pact forces which installed a more Soviet friendly government and instituted crackdowns on freedom of expression, dissident movements, etc. It had become clear that there was no political autonomy in Eastern Europe, only Soviet domination. Whether the rejection of Soviet authority took the form of violent uprisings and revolution (as in Hungary) or peaceful and lawful organization and exercise of political power (as in Czechoslovakia) the result was the same: re-assertion of Soviet control through military occupation and political repression.

Meanwhile, throughout the '50s and '60s border controls at the edge of Soviet controlled Eastern Europe became tighter and tighter. The Berlin Wall and inner German border (with minefields, razor wire fences, machine gun nests, and so forth) were erected, and the borders of Czechoslovakia, Hungary, Romania, and Bulgaria were put under tight control, forming what would become known as the "Iron Curtain".

For anyone to say during the Cold War, in 1976, to say that Eastern Europe wasn't under Soviet domination and never would be was either a terrifically astounding miswording of a sentiment or a revelation of even more astounding geopolitical ignorance. For it to be said by someone who had been President and Commander in Chief for two years was practically jaw dropping. One might as well have said that there was no Soviet domination of Moscow and never would be.

Some folks at the time said that the gaffe was so great it may have cost Ford the election, but in reality it didn't make a huge difference in his poll numbers and he was consistently behind Carter in the polls throughout most of the election campaign.