The question was worded with the name of the major party candidates at the time i.e. in 2008, ‘Barack Obama, a Democrat’, and ‘John McCain, the Republican candidate’; in 2012, ‘Barack Obama, the Democrat candidate’ and ‘Mitt Romney, as the Republican candidate’; in 2016, ‘Hillary Clinton, if she is the Democrat candidate’ and ‘Donald Trump, if he is the Republican candidate’; and in 2020, ‘Joe Biden, if he is the Democrat candidate’ and ‘Donald Trump, the Republican candidate’.
Dotted line indicates change in mode: see Methodology.