Reagan was a properly synecdochal president while Bush II was a metonymic one. The evidence is in the electoral returns and the subsequent treatment. Following Diane Rubenstein, Reagan was a perfect master signifier because he was total nothingness that nevertheless was able to signify Americaness owing to his election. Ergo Reagan was beloved because he could properly demonstrate the hollowness of the presidency without revealing its hollowness.
I'm moving fast but: assume phallic function=master signifier. "For the phallus is a signifier, a signifier whose function, in the instrasubjective economy of analysis, may lift the veil from the function it served in the mysteries. For it is the signifier that is destined to designate meaning effects as a whole, insofar as the signifier conditions them by its presence as a signifier"
Reading back to the essay on "Logical Time": The master signifier is the master signifier by its virtue of having been chosen, much as the move towards the door is predetermined by the prisoners in a moment which exists outside of time yet nevertheless gives rise to the logics that can have explained retroactively the why of what occurred.
thus the President is the President by virtue of his/her having been selected as such, and his/her status must have an explanation. Taking Chad's point advisedly, that the "Time" essay is really about the moment in which the Ego is thrust into the symbolic and how the Ego has an account of its occurrence despite the fact that it seems outside of spatial reasoning, we can apply this reasoning to the election of 2000. With Bush II as the president, an account is needed to account for why he is the President in order to explain the anti-democratic (and republican, hat tip to MKF) peculiarity that explains his rise.
If the Presidency designates meaning effects as a whole, then it will have to look as though the President was legitimately in that position in the first place- hence the strong electoral victory in 2004.