In my latest reading of The Great Betrayal: How American Sovereignty and Social Justice Are Being Sacrificed to the Gods of the Global Economy,
 Pat Buchanan disputes the idea that the Smoot-Hawley tariff during the 
Hoover Administration exasperated the Great Depression.  I thought it 
was cool (and funny) that Buchanan quoted the Ben Stein character's 
promulgation of this idea in the movie Ferris Bueller's Day Off (see here for the scene).
Buchanan
 does not think that Smoot-Hawley significantly impacted enough of the 
economy for it to be blamed for the deepening of the Great Depression.  He states on page 247: "...how much adverse affect could Smoot-Hawley have had on the U.S. economy as a whole, when
 total imports in 1930 added up to only 4 percent of the GNP and 
Smoot-Hawley applied to only a third of that, or to 1.3 percent of the 
GNP?  Is it conceivable that an increase in tariffs on 1.3 percent 
of the GNP triggered the collapse of five thousand banks, wiped out 
five-sixths of the stock market, caused a drop of 46 percent in the GNP,
 and sent unemployment souring to 25 percent?"
Buchanan does not cite sources for these statistics, as far as I could tell.  But he does have charts that are based on the Historical Statistics of the United States and the Statistical Abstract of the United States,
 and these show that the Smoot-Hawley tariff was not as high as many 
other tariffs in U.S. history, tariffs that (Buchanan argues) coincided 
with economic prosperity.  Buchanan also refers to economist Ravi Batra's
 claim that, from 1929-1933, domestic demand fell by a far greater 
percentage of GNP (98.5 percent) than net exports (1.5 percent).  
According to Batra, it's absurd to blame the deepening of the Great 
Depression on trade policy, when the vast bulk of the problem was in the
 area of domestic demand.  And Buchanan refers readers to Alfred Eckes, Jr.'s Opening America's Markets,
 which (according to Buchanan) presents evidence against the 
anti-Smoot-Hawley rhetoric.  Eckes was chairman of the International 
Trade Commission during the Reagan Administration. 
But 
did not other countries retaliate against American exports due to the 
Smoot-Hawley tariff?  According to Buchanan, countries in Europe, Japan,
 India, Australia, and New Zealand were going protectionist way before 
Smoot-Hawley.  Buchanan refers to The Growth of the International Economy, 1820-1960, by A.G. Kenwood and A.L. Lougheed, and Corelli Barnett's The Collapse of British Power. 
 Buchanan notes, however, that the League of Nations helped bring about a
 lowering of tariffs "in almost all developed countries in 1928 and 
1929" (page 250).  That makes me wonder: couldn't the countries have reinstated protectionism after the Smoot-Hawley tariff, in response to it?
According to Buchanan, the Great Depression was deepened by at least three factors:
 the effects of the stock market crash in 1929, which included the 
wipe-out of people's savings and the disappearance of one-third of the 
United States' money-supply; the Federal Reserve doing "nothing to stop 
the hemorrhaging or to replace the lost lifeblood of the American 
economy" (page 249); and Herbert Hoover's dramatic tax increases.
See here for another perspective on Smoot-Hawley.