Thursday, June 21, 2007

Bloomberg Hurts Giuliani More Than Clinton

Michael Bloomberg's possible entry into the presidential race appears to hurt Giuliani more than Clinton, based on analysis of data from SurveyUSA's recent polling in 16 states. While support for both candidates declines when Bloomberg is included in the vote question, Giuliani's support declines by an estimated 1.7 more percentage points than does Clinton's.

The SurveyUSA data were collected by interactive voice response (i.e. "automated" survey) in AL, CA, IA, KS, KY, MA, MN, MO, NM, NY, OH, OR, TX, VA, WA and WI June 8-10, 2007 with approximately 500 respondents in each state. Respondents were first asked
If there were an election for President of the United States today, and the only two names on the ballot were Republican Rudy Giuliani and ... Democrat Hillary Clinton, who would you vote for?
The survey then asked
OK, now imagine that the election for president was a 3 way contest, and the 3 candidates on the ballot are Republican Rudolph Giuliani, Democrat Hillary Clinton, and Independent Michael Bloomberg.

(SurveyUSA asked about other candidate pairs as well, but the focus here is only on the Giuliani-Clinton-Bloomberg effects.)

Adding a third option to any vote question should be expected to draw support away from both candidates in the two candidate only form. In the figure above, that means we would certainly expect the data points to fall below the black 45 degree line in the figure, meaning candidates do worse with three candidates than with two. That obviously occurs in the figure.

The differential impact on the candidates is the more crucial point. If both candidates are equally affected, we'd expec the red and blue points in the figure to mix together more or less randomly in the plot. If one candidate is more damaged than the other, then the blue and red points should separate with one generally closer to the 45 degree line than the other. That's what we see.

Generally the Clinton (blue) points are above the Giuliani (red) points. If we take the simple average changes, Clinton loses an average of 3.6 points when Bloomberg is added, while Giuliani loses an average of 5.2 points. When we do a slightly fancier regression estimate, the net loss hurts Giuliani by 1.7 points more than it does Clinton. That difference is visible in the chart as the gap between the red and blue estimated regression lines.

As a Republican until this week, Bloomberg could be expected to draw more from the Republican than the Democratic candidate.

Of course this is all hypothetical with a mayor who says he expects to serve out his term. But you have to admit that an "all New York" three way race would have its charms.