A recent Washington Post piece, Why Republicans feel so good about the 2014 midterms, in 1 chart, there is a (rather poor) chart showing the difference between Obama's 2012 election vote percentage and his current approval rating percentage, as a leading indicator for the 2014 elections. I thought it strange to compare the two percentages, but the columnist only glosses over the "theoretical" possibility of someone whose vote in an election doesn't match their approval rating.
Is there precedent for comparing approval ratings and election results? Or for doing so with adjustments. Surely past approval ratings are available, even for a more comparable time (six months before the election), and would support a similar if slightly weaker conclusion.