A Display of Democratic Unity as Thompson Cedes to de Blasio As Thompson Exits, Cuomo Looks Ahead: As William C. Thompson Jr. withdrew from the mayoral race, leaving Bill de Blasio the Democratic nominee, Gov. Andrew M. Cuomo urged Democrats to fight together to retake City Hall. Bill de Blasio, completing a stunning political turnaround, assumed the role of standard-bearer of New York City's Democratic Party on Monday, as his last remaining primary rival exited the mayoral race and he turned his attention to a fight against a Republican opponent with a starkly different vision. Joseph J. Lhota, the Republican nominee for mayor, said Monday that Mr. de Blasio wanted radical change for New York. In a coronation of sorts on the steps of City Hall, the state's Democratic leaders, eager to retake the mayor's office after almost two decades out of power, feted Mr. de Blasio as a bold new messenger for their party - even as they praised William C. Thompson Jr., the second-place primary finisher who had held out nearly a week before announcing his withdrawal. "There is nothing more beautiful than Democratic unity, and thank you for it," a beaming Mr. de Blasio said, as Mr. Thompson, his face frozen in a smile, stood behind him. It was a triumphant moment for Mr. de Blasio, until recently a little-known liberal, who overcame doubts from many Democrats about his candidacy. But he almost immediately faced sharp attacks from the Republican nominee, Joseph J. Lhota, who held his own event on Monday and denounced Mr. de Blasio's left-leaning platform as extreme. Arguing that Mr. de Blasio would "tear down" the city's progress of the last two decades, Mr. Lhota said New Yorkers would be choosing between two distinct paths in November. Still, mindful of an electorate seeking a new direction after 12 years with Mayor Michael R. Bloomberg, Mr. Lhota was careful to separate himself from the current administration, saying he wanted to bring his own version of change to City Hall. "Bill de Blasio's change is radical," Mr. Lhota told reporters at a news conference in Central Park just off Fifth Avenue. As the contours of the general election came into focus, Mr. Thompson, the runner-up in the 2009 race for mayor, was again left reflecting on what went wrong. Calling the coming election "bigger than either one of us," Mr. Thompson offered Mr. de Blasio his endorsement at City Hall. But he was far more animated in criticizing the city's Board of Elections, saying it was "a disgrace" that no official count had been determined nearly a week after the primary. Mr. Thompson, who received 26 percent of the vote to Mr. de Blasio's 40 percent in the initial, unofficial results, had hoped that as lever voting machine results were double-checked, and tens of thousands of uncounted paper ballots were tallied, Mr. de Blasio's total might dip below the 40 percent threshold that would prompt an automatic runoff. With a completed count still days away, Mr. Thompson, facing strong pressure from Democratic leaders and defections by some of his important endorsers, chose to end his bid. "We don't know how many votes I got, or even how many votes were cast," said Mr. Thompson, who stood at a lectern emblazoned with a "Bill de Blasio for Mayor" banner. Describing himself in a state of political limbo, he said it would be "impossible" for him to mount a fight without knowing whether a runoff would take place. When are they going to finish? Mr. Thompson said to reporters, referring to the elections board. Without a final count, he added, "it's such a disadvantage - it just isn't fair." Mr. Thompson's remarks were a rare moment of discord at an event meant to showcase Democratic unity and energy ahead of the first mayoral election in years that the party is confident it can win. In a sign that party leaders believed it was critical to close ranks around Mr. de Blasio, the rally was presided over by Gov. Andrew M. Cuomo, who made his first appearance at City Hall since being elected governor in 2010. "Bill is going to lead this city in the great progressive Democratic traditions that made this the greatest city on the planet," Mr. Cuomo said, with Mr. de Blasio at his side. Al Baker, Michael Barbaro and David W. Chen contributed reporting.