FDR hit the ground running for the White House when he was elected governor of New York in 1928. When Al Smith was trounced in the 1928 Presidential race, it was clear that a protracted campaign was needed.
Yes, the stock market crash hit in 1929 and FDR used it as a bludgeon to hit Hoover hard. FDR was considered a fiscal conservative right up until he became governor. He was a rebel within his own party, opposing He took Hoover's advice and vastly expanded the state's investment in just about every area (see my earlier comment about the exploding deficit under FDR in NY State).
In 1930, FDR still had his conservative tendencies. I find it funny that his early battles against the Tammany Hall corruption (due to its misuse of public funds not to mention its election tactics) ended with FDR's huge expansion of the federal sphere of influence and the inevitable waste that follows such a bureaucracy.
FDR was the originator of the strategy used by Bill Clinton in the 1990s: triangulation. Take your opponent's position and make it your own. There is no doubt that FDR was a remarkable politician, not to mention a very charismatic man, and a man of unmatched personal strength.
An interesting thing happened after the election of 1932: despite FDR's pledge of a "new deal for the American people" he embraced Hoover's big government solutions to the Depression. Hoover had abandoned the laissez faire politics of his predecessors.
Going back a bit farther: there is little doubt that Harding was laissez faire. His term lasted only two and a half years, however. His successor, Silent Calvin Coolidge who uttered the famous: "The business of America is business". Coolidge was definitely pro-business and a tax cutter, as demonstrated by the
Revenue Act of 1926. He lowered taxes and the national debt at the same time (the economy boomed throughout most of the 20s). Coolidge was not so laissez faire as a governor, but federalism was still alive then and state and local officials were expected to intervene in certain market issues (child labor, minimum wages, etc.).
Hoover was a progressive, but also a problem solver. He liked tinkering with things to get them to work: he was an engineer. Remember, however, that the stock market crash occurred only 6 months into his term. He certainly did not cause it. Some argue that it was Congress's discussion of Smoot Hawley that was to blame. After the laissez faire 20s, a protectionist measure like Smoot Hawley would dramatically change the economic landscape (and it did).
Many economists blame the Federal Reserve for the Depression. The Fed was created under Woodrow Wilson and was certainly not a laissez faire approach. The President has little control over the Fed and laissez faire laws are not impossible with an interventionist Fed.