Why are some Presidential candidates so intensely disliked?

I was wondering if anyone ever ran a regression to explain why certain politicians are disliked so intensely. One variable that I think would explain a lot is simply how long they have been on the national stage. For the Democrats, that variable would be positively correlated with their ratings. For Republicans, McCain is definitely an outlier. Possibly length of time and whether they take positions normally associated with the other political party. In any case, if Hillary wins the Democratic nomination, the election battle will be fought over a very narrow group of voters.

Which of these prospective presidential candidates would you never vote for?

Newt Gingrich 53%

Hillary Rodham Clinton 46%

Al Gore 43%

Mitt Romney 39%

John Edwards 35%

Barack Obama 33%

John McCain 32%

Rudy Giuliani 31%

Source: Zogby International
Methodology: Telephone interviews with 1,028 likely American voters, conducted from Mar. 7 to Mar. 9, 2007. Margin of error is 3.1 per cent