As the two political parties go to the voters today to try to decide on their nominees, the third drama of the campaign is rapidly coming to a head — whether Mayor Bloomberg is going to enter the race. The amount of time he has taken making decision fits with his methodical ways, and the electoral calendar has its own logic. By our lights the thing for him to remember at this juncture is that there is no upside in not running and all kinds of upside if he does run, particularly because he represents a swatch of voters who are still having a hard time getting comfortable with the other candidates.

It is true that some of the mayor’s allies, such as Senator Lieberman, are saying that the emergence of Senator McCain as the Republican frontrunner makes a Bloomberg presidential run less likely. Mr. Bloomberg has spent the past weeks watching political allies endorse his rivals. Mr. Lieberman, Governor Schwarzenegger, and Governor Pataki threw their backing to Senator McCain, as did Senator Danforth. Governor Napolitano and Senator McCaskill endorsed Senator Obama. Robert Kerrey endorsed Senator Clinton.

Yet the mayor differs sharply with Mr. McCain — and, for that matter, Mr. Romney in his current iteration — on issues such as abortion, gun control, and same-sex marriage, and is a bigger proponent of federal infrastructure spending than is Senator McCain. Unlike either Messrs. McCain or Obama, Mr. Bloomberg has significant, successful private sector experience. Unlike either Messrs. McCain or Obama, who are lawmakers, Mr. Bloomberg has recent executive experience in government, balancing budgets and running an organization with hundreds of thousands of employees and dealing with a legislature with an itch to tax and spend.

There is a sense that the mayor has already waited too long, in sharp contradistinction to, say, Mr. Obama, who just jumped right in. By staying out of the race for so long Mr. Bloomberg may have avoided the problem that befell Mayor Giuliani and, to a lesser extent, Senator Clinton. But by the end of today, the fray could be thinned out and the nature of the opportunity that beckons Mr. Bloomberg could be clearer. Certainly the candidates left standing will be seen in a harsher light.

Mr. McCain’s self-righteous, and somewhat angry, streak will start to look less attractive, as will his effort to demonize the pharmaceutical industry, as the Wall Street Journal pointed out over the weekend. Senator Obama’s campaign for “change” will start to wear thin as it becomes clear that what he is offering as far as policy is not change but the same old Democratic tax-and-spend approach at home, combined with a program of surrender and retreat abroad.

If Senator Clinton is the nominee, her effort to bring America back to the 1990s would contrast with a future-looking program of Mr. Bloomberg. Mr. McCain has been moving away from his own position on immigration so much that it leaves room for Mr. Bloomberg to run as the pro-growth candidate on immigration.

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By our lights, all the logic is for the mayor to get into the race. He has considered a run for so long, so carefully, and so publicly that a failure to throw his hat into the ring will overshadow all his achievements as a businessman, as mayor, and as a philanthropist. It won’t cost him his honor not to run, but even to run and come in third would be better than not running. We say that, as we have for so long, because there are issues of economic growth and open-ness on which he can speak with a unique, New York perspective at a time when our country could yet fall into a terrible economic retreat. And because without him, the field is incomplete. So it’s a time for the mayor to remember the old adage about no guts, no glory, and give the country everything he’s got and let the people decide.