"Right" & "Left" in America are essentially variations of liberalism (in a broad sense - freedom, democracy, human rights, etc.), which causes some difficulty in properly answering the question. The U.S. was basically founded on "right-liberal" principles, but "left-liberalism" has become more influential as the result of the influence of progressivism. Republicans are (generally) right-liberals while Democrats are (generally) left-liberals. In that context, the U.S. is basically centrist, moving leftward. In the broader context of Western/European history, the U.S. is undoubtedly left-leaning, yet not so much as most modern European liberal democracies which have been much more deeply influenced by socialism & communism.