Perhaps it's a factor of Hillary Clinton's consistent lead over Donald Trump in both national polls and in expert assessments of the likely breakdown of the electoral vote in November's presidential election. Or perhaps President Obama has simply decided that there is a line he will not cross when it comes to supporting his preferred successor. But whatever the cause, the president showed once again this weekend that even in the face of opposition from his own party's presidential nominee, he is not going to budge on the question of the Trans-Pacific Partnership trade deal.