Main article: New York City mayoral election, 2005

Bloomberg's election marked the first time in New York City history that two different Republicans had been elected mayor consecutively. New York City has not been won by a Republican in a statewide or presidential election since 1924. He is considered a social liberal, who is pro-choice, in favor of legalizing same-sex marriage and an advocate for stricter gun control laws.

Despite the fact that 68 percent of New York City's registered voters are Democrats, Bloomberg decided the city should host the 2004 Republican National Convention. The Convention drew thousands of protesters, many of them local residents angry over the Iraq war and other issues. The Police Department arrested approximately 1,800 protesters, but according to The New York Times, more than 90 percent of the cases were later dismissed or dropped for lack of evidence.

Bloomberg was re-elected mayor in November 2005 by a margin of 20%, the widest margin ever for a Republican mayor of New York. Former Bronx Borough President Fernando Ferrer won the Democratic nomination to oppose Bloomberg in the general election. Thomas Ognibene sought to run against Bloomberg in the Republican Party's primary election. Bloomberg's campaign successfully challenged enough of the signatures Ognibene had submitted to the Board of Elections to prevent Ognibene from appearing on ballots for the Republican primary. Instead, Ognibene ran only on the Conservative Party ticket. Ognibene accused Bloomberg of betraying Republican Party ideals, a feeling echoed by others.

Bloomberg opposed the confirmation of John Roberts as Chief Justice of the United States. Though a Republican at the time, Bloomberg is a staunch supporter of abortion rights and did not believe that Roberts was committed to maintaining Roe v. Wade.
In addition to receiving Republican support, Bloomberg obtained the endorsements of several prominent Democrats: former Democratic Mayor Ed Koch, former Democratic governor Hugh Carey, former Democratic City Council Speaker Peter Vallone, his son, Councilman Peter Vallone, Jr., former Democratic Congressman Floyd Flake (who had previously endorsed Bloomberg in 2001), and Brooklyn Borough President Marty Markowitz.

Some of the policies Bloomberg advocates parallel those of either the Democratic or the Republican party platform. He is socially liberal, supporting abortion rights, gay marriage, gun control, and normalization of the status of illegal immigrants, for example. On economics, foreign, and domestic issues, he tends to be conservative. He opposes a timeline for withdrawal from Iraq, and criticizes those who favor one. Economically, he supports government involvement in issues such as public welfare and climate change, while being strongly in favor of free trade, pro-business, and describing himself as a fiscal conservative because he balanced the city's budget.