As speaker, Ms. Quinn has been a forceful counterpart to Mr. Bloomberg, and has turned the Council from a collection of rambunctious, ill-directed egos into a forceful and effective legislative body. In wrestling with budgets she has shown restraint that runs counter to lesser political instincts. She fought, for example, for a Bloomberg plan to keep a year’s surplus as a rainy-day fund. There was fierce opposition from Council members who wanted to spend the money. Ms. Quinn was right, and the city had a cushion when the recession hit.

Mr. Bloomberg has raised expectations that hard decisions should be made on the merits — that the city needs a mayor who is willing to say no. More than with the other candidates, that description fits Ms. Quinn. As an early leader in the campaign, with a target on her back, she has faced anger and derision without wavering. We admire her staunch support for the city’s solid-waste management plan, which is good for the whole city but bitterly opposed in some neighborhoods. She has been willing to challenge the mayor’s misjudgment and insensitivity, as when he tried to require single adults to prove their homelessness before they were allowed to use city shelters.

Mr. de Blasio has been the most forceful and eloquent of the Democrats in arguing that New York needs to reset its priorities in favor of the middle class, the struggling and the poor. His stature has grown as his message has taken root — voters leery of stark and growing inequalities have embraced his message of “two cities.” He has ennobled the campaign conversation by insisting, correctly, that expanding early education is vital to securing the city’s future. And yet, Mr. de Blasio’s most ambitious plans — like a powerful new state-city partnership to make forever-failing city hospitals financially viable, or to pay for universal prekindergarten and after-school programs through a new tax on the richest New Yorkers — need support in the State Capitol, and look like legislative long shots. Once a Mayor de Blasio saw his boldest ideas smashed on the rocks of Albany, then what?

Mr. Thompson, meanwhile, who nearly defeated Mr. Bloomberg four years ago, has run a thoughtful campaign grounded on the insights he gained in important elective and appointed posts in New York City. A former president of the old Board of Education, Mr. Thompson argues that he is the best candidate to fix the city schools, but his close ties to the United Federation of Teachers, not always a friend of needed reforms, give us pause. The teachers’ union is one of the municipal unions itching for retroactive pay raises in contracts that expired under Mr. Bloomberg and need renegotiating.

For all the growing testiness of the campaign, the Democrats share much common ground. All agree on equality, opportunity and fairness. They concede that the best of the Bloomberg years — the economic diversification and growth, the astounding drop in crime, the transit innovations, the greener and cleaner public spaces, and big plans for the future — must be preserved. And they agree that the worst must be corrected — starting with the Police Department’s unconstitutional use of stop-and-frisk, which has abused and humiliated hundreds of thousands of innocent New Yorkers.