Bill de Blasio opens the final phase of his bid for New York City mayor with a daunting lead over his Republican opponent, Joseph J. Lhota, who appears to face a tougher path to victory than even his most pessimistic supporters imagined.

Mr. de Blasio, whose last remaining rival for the Democratic nomination conceded this week, enjoys support from 65 percent of likely voters, compared with 22 percent who back Mr. Lhota, according to a WNBC/Wall Street Journal/Marist poll released on Tuesday.

That 43-point gap would be intimidating for nearly any candidate. But Mr. Lhota must also win over an electorate in a city in which registered Democrats outnumber registered Republicans by nearly six to one, and the low poll numbers could further depress his already lackluster fund-raising.

There are copious caveats. The general election has just begun. Mr. Lhota, a former chairman of the Metropolitan Transportation Authority who was a deputy mayor in the Giuliani administration, is not well known by the public. A finely tuned television advertisement can upend a race. And Mr. de Blasio, currently the city’s public advocate, is riding enormous momentum and news media attention after his primary victory.