“For Ed Mangano, public service was self-service,” the lead prosecutor, Catherine Mirabile, said during closing arguments at the latest trial. “From the moment he took office, he cashed in the power to benefit himself and his wife.”

That assessment could have been made for many members of the Nassau County Republican machine in recent years, as corruption scandals have picked off its leaders, who time and time again have been shown in courtrooms to have peddled their influence to line their pockets, violating the law.

It is a group that once wielded extraordinary influence and had a lock on elected offices for decades. But a culture of corruption has eviscerated Republican control of the county, and each new conviction has helped shift more power to Democrats, political strategists said.

“When you are as powerful an electoral majority as the Nassau Republicans used to be, you tend to get a little arrogant, and you tend to believe that you can do things and not get caught,” said Steve Israel, a former Democratic congressman representing Long Island. “That has clearly caught up with the Republican machine in Nassau.”

Today, Laura Curran, a Democrat, holds Mr. Mangano’s former position, and Democrats now make up a majority of party-affiliated voters in a county that once was the apogee of Republican control of America’s suburbs.

Last November, Long Island was credited with returning the State Senate to the Democrats for the first time in decades, and Nassau County was pivotal in that effort, electing Democrats to three seats formerly held by Republicans.

Part of the change is demographic. Lawrence Levy, the executive dean of the National Center for Suburban Studies at Hofstra University, said an influx of younger, minority and immigrant residents is one reason the city’s suburbs, once strongholds of Republican power, have begun to wobble, a trend that he said bears out countrywide.