FDR's inspiration to enter politics was the success of his cousin, Teddy Roosevelt, who became president when Franklin was still in school. While was a law clerk after graduation from Columbia, FDR boasted to his coworkers that he had already planned his path to the presidency, a path that greatly resembled his relative's. Indeed, politics came calling even before Roosevelt had a chance to act on his own. John Mack, the Democratic District Attorney in New York, came to the office to get some papers signed and offered Roosevelt the chance to run for the Assembly Seat that would be left vacant when a current Assembly member ran for State Senator. FDR was so elated by the opportunity that when the Assembly member decided to stay in his position, he ran for the State Senate seat himself. Roosevelt was a staunch Democrat, like his father, although he had voted for Uncle Ted in his last election. He rented a Maxwell touring car and campaigned all around the state with Richard Connell, an inspired speaker and perennial Democratic candidate for the congressional seat held by Republican Hamilton Fish. They stopped and spoke to every man they saw in their drive through the rural areas of this district of New York.

Roosevelt defeated the incumbent 15,708 to 14,658, running ahead of him in the rural areas where he had campaigned so enthusiastically. FDR had won, but his win was a part of a Democratic landslide all over the state. Even Dick Connell, who had campaigned unsuccessfully for many years, won the seat that he had pursued for so long. Roosevelt's first victory was as much a reflection of his good luck as his hard work: the Republican Party, having split into two because of Teddy Roosevelt's clash with Taft's policies in office, did not put up a good fight in New York. This brand of sheer luck was to follow FDR throughout his political career, helping him to always be in the right place at the right time.

In office, FDR was immediately singled out from the other new politicians because of the power of the Roosevelt name and wealth. He bought a house within walking distance of the capital, suitable for entertaining as well as for accommodating his whole family–something most young legislators could not afford. FDR made headlines almost immediately upon entering office by fighting the Tammany Hall bosses who had control over the politics of New York. He went against their choice for U.S. Senator, William Sheehan. FDR was elected chairperson of the insurgents, who refused to vote in the caucus for Senator, on the principle that their votes would be smothered in the caucus.

FDR received national attention for his role in the fight, especially from Woodrow Wilson, who was fighting a similar battle as governor of New Jersey. Teddy Roosevelt and the New York Times also took kindly to FDR's role in the affair. The insurgents nonetheless faced a great deal of resistance in their fight against Tammany Hall, and it was expensive for Congress to be in session without accomplishing anything. All former patronage was now denied them. Charges were made that they were anti-Irish and anti-Catholic. Poorer legislators were threatened with called loans and foreclosures. Eventually, Tammany Hall's Boss Murphy outwitted the insurgents. He submitted the name of the former Supreme Court Justice James O'Gorman, who was a leader in Tammany Hall, for the Senatorial seat, thus replacing the old candidate with someone who was more an enemy to Progressive Democrats than the original candidate. Despite the obvious win for the Tammany Hall bosses, the affair was still remembered by his associates and by the public as an episode in which FDR "twisted the tail of the Tammany tiger," a testimony to his political savvy.

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