It is remarkably uncommon for a US president to fail to be re-elected. It has happened just twice in the long lifetime of Joe Biden: with Jimmy Carter in 1980 and George H.W. Bush in 1992. On Tuesday, however, it looks likely that it will happen again. It is not just that Donald Trump is trailing badly and consistently in the national polls — he was behind in 2016 but won nonetheless — it is that his support seems to be draining most in his own heartlands.