While Roosevelt fought with his Rough Riders in Cuba, another man began preparations for a more political battle in Roosevelt's home state of New York. This man was Senator Thomas Platt, the boss of New York State's Republican political machine, who needed to find a new governor for the 1898 election. He preferred the incumbent–a party man by the name of Frank S. Black–but the current governor had been involved in too many embezzlement scandals, and Platt realized his chances of reelection were slim. With this in mind, he began quietly searching for a new candidate and found Theodore Roosevelt, the recent war hero and former Assistant Secretary of the Navy. Platt telegraphed Roosevelt in Cuba and asked him if he would accept the nomination. Roosevelt replied he would be more than delighted to accept, and upon returning to New York, began campaigning for the gubernatorial office. Many of Roosevelt's former reformist friends within the Grand Old Party felt betrayed and condemned Roosevelt for becoming part of the machine.

Roosevelt did not abandon his friends entirely, however. After a narrow victory over the Democratic candidate, Augustus Van Wyck, Roosevelt began heeding his own political intuitions rather than Boss Platt's. Boss Platt was not pleased, and the two butted heads when Roosevelt refused to hire Platt's choice for Public Works Commissioner, choosing his own instead. Roosevelt also voiced support for a bill granting tax cuts to public service organizations–a bill conservative Republicans detested and fought to block. The business interests of New York were particularly outraged, as many of them had donated money to Platt's machine to avoid this type of legislation. They accused Roosevelt of being too reformist, and feared that the Progressive governor would destroy the entire machine within a matter of years. Once elected, however, there was little Platt could do to stop Roosevelt.

Fortunately for Platt–and eventually for the rest of the nation–an opportunity to get rid of Roosevelt did present itself in the form of the Presidential election of 1900, in which Republican President William McKinley sought a new Vice Presidential running mate. Platt seized the chance and nominated Roosevelt for the Vice Presidency, a position that had little political power. He figured that McKinley would benefit from Roosevelt's war hero status, Roosevelt himself would be stripped of much of his political weight, and, most importantly, New York would once again be under Platt's own control. Roosevelt, for his part, did not wish to accept the Vice Presidential nomination because he believed it would destroy his public office career, render him politically inactive, and strip him of his political influence. He was the popular choice, however, and at the 1900 Republican Nominating Convention in Philadelphia, twenty thousand representatives chanted "We want Teddy, we want Teddy" until he finally reluctantly conceded and accepted the nomination.

Despite his reservations, Roosevelt campaigned with every ounce of his fighting spirit. During this period in American politics, decorum dictated that the Presidential candidates abstain from too much campaigning so as not to cloud the loftiness of the office. McKinley, a solemn and well-bred man from the East, therefore did not campaign. He left this job to Roosevelt and Mark Hanna, McKinley's Republican advisor and the man who engineered the entire election effort. During the campaign, Roosevelt traveled more than twenty thousand miles on his special campaign train throughout the U.S. and spoke in front of nearly three million Americans. He railed against the Democratic candidate William Jennings Bryan, a man he personally and politically disliked for his leftist ideas. Bryan and the Democrats cried for printing more money backed by silver and attacked McKinley and Roosevelt for their imperialist Spanish-American War. Roosevelt responded that the gold standard was sound and that the war to liberate subjugated peoples had been morally righteous. In the end, McKinley and Roosevelt defeated Bryan by nearly a million popular votes. McKinley went on to win easily in the Electoral College.

Roosevelt thoroughly detested the Vice Presidency. He found it dull and a waste of his time. His duties as Vice President required him to preside over the Senate, but by the time he was elected, this job only lasted five days before the Senate session ended. Furthermore, most of those days were spent confirming McKinley's new appointments. Within a short time, Roosevelt retired to ""Sagamore Hill"". He made plans to continue the law studies he had dropped earlier in his career, figuring that at the end of his term as Vice President he would be out of politics for good. He believed that studying law would provide a valuable distraction from his boredom and might be helpful in the future.

Roosevelt never had the chance to return to his law studies, however. Within six months of his term, he received grave news while on a trip in Vermont: President McKinley might be dying. On September 6, 1901, the President had been shot while receiving guests at a reception at the Pan-American Exposition in Buffalo, New York. The attacker, an anarchist named Leon Czolgosz, held a pistol underneath a handkerchief in his hand. He stood in line to shake hands with McKinley, and when the President stretched out his hand to greet Czolgosz, he was shot twice in the stomach. Roosevelt rushed to Buffalo on a special train and was briefed on McKinley's emergency surgery. When Roosevelt arrived, the President's doctors assured him that McKinley would make a full recovery. Within a week, however, his conditioned worsened, and in the early hours of the morning of September 14, 1901, McKinley died.

Theodore Roosevelt was sworn in as president in Buffalo later that day. Many Republicans and Wall Street bankers took this as an ill omen, as McKinley had always supported the party's causes and had acted favorably towards the business interests, while the unpredictable Roosevelt was a loose cannon and a known reformer. In the wake of McKinley's assassination, Roosevelt assured party leaders and financiers that he would keep a low profile. Hanna advised him to take it slowly. Roosevelt made few changes to the McKinley administration; he kept John Hay as Secretary of State, Elihu Root as Secretary of War, and Philander Knox as the Attorney General, among others he retained. Roosevelt could not play the role of dutiful party man for long, however. As he had proved many times in the past, he was his own man, and after the initial shock of the assassination settled, Roosevelt began making a new name for himself as President of the United States.

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