WASHINGTON—President Obama on Wednesday afternoon announced he was nominating Caroline Kennedy as the US ambassador to Japan, an appointment that would catapult the only surviving child of slain President John F. Kennedy to a dynamic diplomatic post.

Kennedy, who has been rumored to be in line for the position for months, would be the first female American ambassador in Tokyo.

Kennedy has a long history with President Obama, having endorsed him at a crucial point in the 2008 Democratic primary contest. She also cochaired Obama’s 2008 search for a running mate and cochaired his 2012 reelection effort.

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Obama announced Kennedy’s nomination along with a handful of other appointments, lumping her in with other ``fine public servants’’ who he said will serve the nation well.

Although ambassador to Japan is seen as a desirable position, it is not an easy one. Kennedy would enter at a time when the rise of China and threats of renewed conflict on the Korean peninsula will require keen negotiating skills.

If she is confirmed to the post, her boss would be Secretary of State John F. Kerry, the former Massachusetts senator.