[deport]

This table details the standings of the candidates on Labor Day. Winning candidates are highlighted in bold letters. 

	Sample	Republican candidate	Democratic candidate	Third-party candidate	Third-party candidate
2000 Sep 4		Bush	Gore	Nader	Buchanan
2000 Sep 4-6	LV	44%	47	3	*
2000 Sep 4-6	RV	41%	47	3	1
1996 Sep 2		Dole	Clinton	Perot
1996 2-4	LV	36%	53	5
1996 2-4	RV	35%	54	5
1992 Sep 7		Bush	Clinton
1992 Sep 11-15	RV	42%	51
1988 Sep 5		Bush	Dukakis
1988 Sep 9-11	RV	49%	41
1984 Sep 3		Reagan	Mondale
1984 Sep 7-9	RV	55%	44
1980 Sep 1		Reagan	Carter	Anderson
1980 Sep 12-15	RV	37%	41	15
1976 Sep 6		Ford	Carter	McCarthy
1976 Sep 24-26	RV	40%	51	4
1972 Sep 4		Nixon	McGovern
1972 Sep 22-25	RV	61%	33
1968 Sep 2		Nixon	Humphrey	Wallace
1968 Sep 3-7	RV	43%	31	19
1964 Sep 7		Goldwater	Johnson
1964 Sep 18-23	RV	32%	62
1960 Sep 4		Nixon	Kennedy
1960 Sep 9-14	RV	47%	46
1956 Sep 3		Eisenhower	Stevenson
1956 Sep 9-14	RV	52%	41
1952 Sep 1		Eisenhower	Stevenson
1952 Sep 6-11	RV	56%	40
1948 Sep 6		Dewey	Truman	Wallace	Thurmond
1948 Sep 10-15	NA	47%	39	3	2
1944 Sep 4		Dewey	Roosevelt
1944 Sep 8-13	NA	45%	50
1940 Sep 2		Willkie	Roosevelt
1940 Sep 5-10	NA	40%	49
1936 Sep 7		Landon	Roosevelt	Lemke	Thomas
1936 Sep 7-12	NA	45%	49	5	1

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[deport]

Gallup's polling history allows us to examine the bounces that have occurred for both major-party candidates in each election since 1964.

Post-Convention Increases in Support, 1964-2004
Election	Candidate (incumbent in boldface)	Bounce (among registered voters)	Convention Order
2004	John Kerry	-1 point -2 points (likely voters)	1st
2000	George W. Bush	8 points 4 points (likely voters)	1st
2000	Al Gore	8 points 8 points (likely voters)	2nd
1996	Bill Clinton	5 points	2nd
1996	Bob Dole	3 points	1st
1992	Bill Clinton	16 points	1st
1992	George Bush	5 points	2nd
1988	George Bush	6 points	2nd
1988	Michael Dukakis	7 points	1st
1984	Ronald Reagan	4 points	2nd
1984	Walter Mondale	9 points	1st
1980	Ronald Reagan	8 points	1st
1980	Jimmy Carter	10 points	2nd
1976	Jimmy Carter	9 points	1st
1976	Gerald Ford	5 points	2nd
1972	Richard Nixon	7 points	2nd
1972	George McGovern	0 points	1st
1968	Richard Nixon	5 points	1st
1968	Hubert Humphrey	2 points	2nd
1964	Lyndon Johnson	3 points	2nd
1964	Barry Goldwater	5 points	1st

[Diddle E. Squat]

So the Labor Day leader has seen his lead shrink in all but 2 of the 14 elections from 1948 on.

-3 in 2000
-9
-3
-1
+7 in 1984
-14
-9
-5
-11
-8
-1
+4 in 1956
-5
-13

Average is a 5 point drop from Labor Day to the election, -6 if you throw out the 1984 anomaly. Average from 1964 on is -5.6, or -7 if you throw out 1984.

But of course this is way too small a sample size to read much into other than a general prediction, just like numerical analysis often fails to accurately predict the winner of football games.