The Empire Slate

Millions of voters change party affiliation every year, with little effect on the trench warfare of American politics. But for the first time since Dwight Eisenhower declared himself a Republican in 1952, it’s possible that one man’s new political label could truly transform a presidential campaign.

When Michael Bloomberg switched his party affiliation from Republican to independent on Tuesday, he coyly opened the door for a historic election in 2008. If Mr. Bloomberg uses his fortune to finance a third-party campaign, and if Hillary Clinton and Rudolph Giuliani win the Democratic and Republican nominations, three serious candidates will call New York home.

Candidates from the same state have faced each other only four times in presidential elections. The first such time, the 1860 race in which Abraham Lincoln defeated a fellow Illinoisan, Stephen Douglas, was the most consequential of all. The other three — involving the New Yorkers Theodore Roosevelt and Alton Parker in 1904, the Ohioans Warren Harding and James Cox in 1920, and the New Yorkers Franklin Roosevelt and Thomas Dewey in 1944 — were also tinged by the intramural rivalry.

What changed with these unusual races? For starters, both candidates needed to fight for their home state, which nominees usually take for granted. (In all four races with candidates from the same state, the winner of the home state also won the election.) Nearby states also became up for grabs. With the candidates fighting to get control of their base, states outside the region usually got less attention. And though the candidates often differed substantially on the issues, they brought to the race the shared values and sensibilities of their states.

As the Lincoln-Douglas battle showed, same-state candidates completely change the political calculus for the whole election, from the nomination process to the general election.

Starting with their famous debates in the 1858 race for the Senate seat from Illinois, Lincoln used Douglas’s prominence to create openings for himself. He worked hard, for example, to discredit Douglas’s argument that states should decide the slavery question for themselves. When Douglas won the Senate seat, Lincoln never stopped fighting. In 1859, he called Douglas “the most dangerous enemy of liberty, because the most insidious one.”

Lincoln’s attacks won him more headlines — and provoked speculation that he might make a good candidate for president.

When Douglas took the Democratic nomination, the Republicans needed a candidate to battle Douglas in Western states like Illinois. As Douglas’s constant antagonist, Lincoln was an obvious choice. If the Democrats had picked someone from, say, New York, the Republicans probably would not have nominated Lincoln.

Lincoln, who knew Douglas’s positions and weaknesses better than anyone, was also perfectly positioned to exploit contradications in his opponent’s platform. The criticism took. The Democratic party splintered. And Lincoln prevailed.

While subsequent elections with same-state presidential candidates have not rivaled the Lincoln-Douglas race in either import or spirit, they have come with ties and entanglements that have given them a special dynamic.

In 1904, Theodore Roosevelt easily beat Alton Parker. The country was captivated by Roosevelt’s reformist zeal. Roosevelt took on every major problem of the times — from trusts to the environment to labor conditions — and projected American might overseas. The twist is that Parker himself had strong reformist bonafides.

Indeed, the outlines of the 1904 election were set seven years earlier, in 1897, when the Republican Party machine of Thomas Platt suffered a devastating defeat in New York City. Platt’s candidates for mayor and many other offices were cast out by an electorate unhappy with party corruption.

One of the surprise Democratic winners that year included Parker, a candidate for chief justice of the state’s Court of Appeals. To rebuild the freshly defeated Republican Party, Boss Platt agreed to give the 1898 gubernatorial nomination to the reform-minded Roosevelt.

Platt quickly regretted his choice. He could not control Roosevelt — the two battled over patronage, trusts and labor. So the boss, in an effort to get rid of his young nemesis, helped to engineer Roosevelt’s selection as President William McKinley’s running mate in 1900. With McKinley’s assassination in 1901, Roosevelt became president.

The two reformers — one volcanic, the other placid — fought for the White House in the next election. Parker’s role was to get bloodied with dignity by the popular young Republican president so the Democrats could regroup for another day.

The situation played out a little differently in 1920, setting up not a reformist presidency but a corrupt one. Warren Harding owed his Republican nomination to James Cox’s selection as the Democratic nominee. “As Ohio goes, so goes the Union,” went the old saying. In the elections since the Civil War, Ohio has voted for winners all but four times. The Republicans did not want to let the Democrats take Ohio without a contest.

With Harding’s nomination, the Republicans neutralized any potential Democratic advantage in Ohio. Harding appealed for a “return to normalcy” after Woodrow Wilson’s activist Democratic presidency, World War I and the bitter fight over the League of Nations.

Without a fellow Ohioan in the race, Cox would have easily won Ohio and built a strong base in nearby states. Cox had a stellar record as governor, with reforms of rural schools, labor, budget and public administration, roads and prisons.

But Midwesterners had a choice and went with the man and party promising a quieter brand of politics. The campaign turned on the national mood of weariness toward Wilson and the Democrats. Harding prevailed.

In 1944, no one thought Thomas Dewey would prevail over President Franklin Roosevelt. But Dewey used his base in New York to battle Roosevelt, making the race closer than many thought possible. Though Dewey won just 12 of 48 states, he was competitive in 14 other key states. Dewey held Roosevelt to less than 53 percent of the vote in New York and four other Northeastern states and four Midwestern states. In most of those states, big-city party machines carried the day for Roosevelt.

If the news media had reported Roosevelt’s failing health, if Dewey had more experience as a governor (he had served less than two years), or if the contest had not taken place in wartime, Roosevelt might have been beatable.

Dewey’s New York career bore the imprint of Roosevelt’s activism. Elected governor in 1942, Dewey developed a Republican version of the New Deal.

Throughout the campaign, Dewey seemed a paler shade of his opponent. He disappointed followers by not attacking Roosevelt over party and labor corruption, court packing, Social Security and military preparedness before Pearl Harbor.

Dewey acknowledged that Roosevelt’s activism, in New York and beyond, shaped his career. “I am a direct lineal descendant of F.D.R. without the personal charm,” he said, wryly referring to the Chicago Tribune’s coverage of his career.

It is unclear what Hillary Clinton, Rudolph Giuliani and (maybe) Michael Bloomberg will have to say about one another’s records in New York. It is clear, though, that each will have a strong chance to build a national campaign. And it’s a pretty fair bet that each will also have to spend time and energy campaigning in a place general election candidates rarely go: Home.

Charles Euchner, the author of Extraordinary Politics: How Protest and Dissent are Changing American Democracy. He is writing a book about suicide at the Golden Gate Bridge.