Mayor Bloomberg’s donation of $125 million to fight cigarette smoking, primarily in developing countries, will certainly save many thousands of lives, possibly millions.

It is an extraordinary gift to an area of public health not traditionally seen as an object of private philanthropy, and hopefully it will encourage other donors to become involved in the prevention of death and disease due to lifestyle choices.

The gift shows the mayor’s unique ability to use his personal resources to accomplish humanitarian and life-saving purposes. This is one factor that differentiates him from the rest of the political world. In this area even the best or most well-known politicians are able to compete.

A person with that influence and dedication will inevitably be considered for the presidency, if he wants to be. That does not mean he will succeed, but he probably has a better chance to reach the White House than he did of reaching City Hall when, as an unknown private citizen, he quietly began his mayoral campaign in 1999.

Mr. Bloomberg faces the same issue he did in his mayoral race: in which party should he compete. When he ran in New York City, he took over the moribund Republican Party and used it as his political base. Six years later, the Republicans have not gained strength here. In fact, for the last two elections they did not even offer a candidate for comptroller or public advocate. The mayor, however, was twice elected on the Republican line, with the support of the Independence Party, claimed to have been cleansed of its radicals, and in 2005 the Liberal Party, who were enabled to qualify by Bloomberg volunteers.

For 2008 the Democratic and Republican nominations appear to be pretty well sewed up by existing contenders or by other party regulars, so Mr. Bloomberg’s candidacy would have to be on an independent line. It is not impossible to qualify for the ballot. Ross Perot did it in all 50 states in 1992 and 1996, polling a remarkable 19,743,821 votes, or 18.9% of the total, the highest vote ever for a third party candidate. Many Perot votes were from people who were dissatisfied with both Bush the first and Bill Clinton. More Perotians are considered to have been Bush supporters than Clintonistas, so his candidacy was instrumental, if not decisive, in Mr. Clinton’s victory with 43% of the vote.

Ralph Nader ran for president three times, his Green Party candidacy peaking in 2000, when he received 2,883,105 votes or 2.7% of the national total, running on the left. The vast majority of Nader votes came from Al Gore rather than from Bush the second. The Nader vote was greater than the margin between Bush and Gore in seven states — Iowa, Minnesota, New Hampshire, New Mexico, Oregon and Wisconsin, and, of course, Florida where the vote totals say it with crystalline lucidity — Bush: 2,912,790, Gore: 2,912,253, Nader: 97,421, Pat Buchanan: 17,412 (including thousands of elderly Jewish voters confused by the butterfly ballot).The electoral vote, Bush: 271 (including 25 from Florida), Gore: 266.

If he should run for president, Mayor Bloomberg would probably receive millions of votes, drawing from both parties but primarily from Democrats. His total would be reduced if it were felt that he had no chance to win, because of many Democrats’ intense hatred of the Republican Party. Nonetheless, the $500 million Mr. Bloomberg is reported to have said he is willing to spend would have substantial impact, particularly in states where television commercials are less expensive than in New York.

Mr. Bloomberg has certain advantages over other independent candidates — he is not crazy like Mr. Perot or ideological like Mr. Nader. Like the last third party candidate elected, Jesse Ventura, governor of Minnesota, he is a man who has been successful in his own field and is widely known by the public.

He is also likely to run against major party candidates who have negatives. For example, if Hillary Clinton were the Democrat and Mayor Giuliani the Republican, there are people who for various reasons might not care to vote for either. Such a campaign would be a trifecta for New York State, better than the subway series of 2000, with all three major candidates being New Yorkers. That combination is unlikely to occur, but as we have learned in politics as in life, anything can happen.

Mr. Bloomberg had another major advantage in the mayoral race. At the end of the day, even if he lost, he would still be a billionaire. If Mr. Ferrer lost, he would return to being a nobody. Even if he is not elected president, Mr. Bloomberg will remain a significant national figure whose substantial resources are likely to be employed for the public good. There is always room for a benevolent plutocrat in our society, and the fact that he has twice submitted himself to the voters and been elected gives him a credential that no other donor possesses. (Arnold Schwarzenegger has the celebrity, and some of the money, but as we know there is a constitutional impediment to his further advancement.)

At the same time, the earthly job of running New York City has required Mayor Mike to make numerous decisions, a few of which are likely to have been in error, since even he does not claim infallibility. He must rely on other people, some of whom are more able than others. He must rely on the information he receives, some of which is likely to be slanted, self-serving, or inaccurate.

Unhappy events may occur for which he will be blamed, either for the occurrence itself, for his failure to prevent it, for the city’s response, or for his own reaction (Rule 23-T: It comes with the territory). That is what it is like being mayor. It is a wonderful job if one knows how to do it and cares enough to do it right.

We think it is a great honor for New York to have three presidential candidates. One came from Massachusetts, one came from Illinois by way of Arkansas, and one came from Brooklyn, with stops in Garden City and Washington, D.C. The three are of different religions, and represent both genders. They are all very intelligent people who have had problems with personal relationships. But haven’t we all.

Mr. Stern is president of New York Civic and a blogger on public issues. He was New York City’s parks commissioner for 15 years under Mayors Koch and Giuliani.