WASHINGTON — President Obama on Wednesday nominated Caroline Kennedy to be ambassador to Japan, moving to give a scion of America’s most enduring political dynasty a diplomatic post that has often gone to political heavyweights.

In naming Ms. Kennedy, whose nomination has been rumored for months, Mr. Obama is keeping with a well-established tradition of rewarding important campaign supporters with plum embassies. He recently put forward big-dollar fund-raisers to be envoys in London, Berlin, Copenhagen and Madrid.

But Ms. Kennedy’s value to Mr. Obama has been less about money than mystique. As the daughter of President John F. Kennedy, her imprimatur on his candidacy in 2008 — along with that of her uncle, Senator Edward M. Kennedy of Massachusetts — elevated Mr. Obama at a crucial moment against his better-known rival, Hillary Rodham Clinton.

And it gave Ms. Kennedy lasting ties to Mr. Obama, something analysts said would come in useful in Japan, where officials and journalists have been buzzing with speculation about what a Kennedy in Tokyo would mean for Japan’s standing in the United States.