Did Communism pose a serious threat to the U.S. in the 1940s and 1950s?Was the threat of Communism just an exaggeration to benefit politicians who...

Depends on your definition of "threat".  There was never
any danger that the US would ever be invaded or become communist from internal
revolution.  While communism became the most popular in this country during the Great
Depression, it has always been a small minority politically, as has
socialism.

But the Soviet Union as a communist empire did
seek to expand its control over more countries, exported armed rebellion to achieve
this, and even had a branch of government, the COMINTERN or Communist International to
pursue this goal.  So some countries in Europe became communist, and others were
threatened militarily during the Cold War, and when communism began to spread to the
Western Hemisphere, Guatemala and Cuba in the 1950s, specifically, we tended to view it
as more of a direct threat.

I think it is more accurate to
say it was a threat to our resources and economic standard of living as opposed to our
very national existence or form of government.