Political dynasties are alive and well from coast to coast — but the biggest modern-day American dynasty is suddenly, dramatically, on the wane.

With Rep. Patrick Kennedy’s (D-R.I.) surprise announcement last week that he won’t seek re-election, it is almost a certainty that come 2011, there will be no Kennedy holding a federal elected office for the first time since 1946 — the year John F. Kennedy was elected to the House.

This election cycle alone has seen several members of the famously competitive clan decide against entering high-profile races. It started in early 2009 when Caroline Kennedy, JFK’s daughter, abruptly removed her name from consideration for a vacant Senate seat in New York.

Equally decisive was former Rep. Joe Kennedy (D-Mass.) opting out of the special election to replace his uncle, the late Sen. Edward Kennedy (D). In a recent interview with the Boston Globe, Kennedy acknowledged that passing on the race “wasn’t the greatest decision I ever made in my life” considering that now-Sen. Scott Brown (R) won the seat that Ted Kennedy held for almost 47 years. It should be pointed out that the late Senator’s widow, Vicki Kennedy, also decided against running for the seat, despite entreaties from some Senate Democrats to do so.

Also this cycle, Chris Kennedy, a son of Robert F. Kennedy who runs the family business at the Merchandise Mart in Chicago, flirted with the idea of running for statewide office in Illinois, but he eventually decided to remain on the sidelines.

In California, Bobby Shriver — the eldest son of Eunice Kennedy Shriver and Sargent Shriver who serves on the Santa Monica City Council — talked openly of running for state attorney general in 2010 but decided against it. His youngest brother, Anthony Shriver, who runs the nonprofit group Best Buddies, is still occasionally mentioned as a possible candidate for Florida governor this year, but it is awfully late in the process and the Democratic establishment has settled on state Chief Financial Officer Alex Sink as its preferred candidate.

Even if they’re not in elective office, dozens of Kennedy cousins who are now in their 40s and 50s are doing well and doing good — and any number could wind up running for public office some day, in any number of states. But it’s somehow fitting that the Kennedy most discussed as a possible candidate this year is now Joe Kennedy III, the former Congressman’s son. Some Massachusetts insiders have suggested that this Kennedy, who is just out of law school, could wind up running for Congress if Rep. Bill Delahunt (D) retires.

It may be that the torch is being passed to a new generation of Kennedys, educated in this century — and spared some of the harsh and unceasing publicity that their parents and grandparents endured for most of their lives.