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UPDATED 5/19/11 1:17 p.m.

U.S. Rep. Mazie Hirono has announced her candidacy for the U.S. Senate seat being vacated by Sen. Daniel Akaka next year.

Hirono, 63, has held Hawaii’s 2nd Congressional District seat since 2007. She previously served as lieutenant governor.

Hirono said in an email Thursday that since Akaka’s announcement that he was stepping down, she has been urged by longtime supporters to run:

Thoughts and well-wishes have come from every island: I’m humbled and inspired by the support I’ve received from Hawaii families.

Hirono’s announcement is sure to shake up the Democratic field for the Senate primary. A recent Honolulu Star-Advertiser poll showed her and former Congressman Ed Case neck and neck in terms of voter preference.

An announcement by Hirono will mean a vacant House seat, and viable candidates are sure to be considering their options.

It also means that Hawaii’s other congresswoman, Colleen Hanabusa, may choose not to run for the Senate next year.

In a statement released this afternoon, Hanabusa said, “I appreciate that Mazie called me beforehand to tell me the news — and I wish her well. I am definitely considering a Senate run, but I am still evaluating all my options. I will make my decision when I feel it’s appropriate.”

Former Gov. Linda Lingle said this week that she would decide by late August whether to seek the Republican nomination.

When Akaka announced earlier this year he was retiring from the Senate, it set of tremendous speculation of a political free for all among Democrats.

Other names mentioned at the time as potential candidates included former Honolulu Mayor Mufi Hannemann and current Lt. Gov. Brian Schatz.

But, some party insiders worried that the crowded field would result in a bloody primary contest, sending the wounded victor to face a strong candidate like Lingle in the general.

With Hirono’s announcement, however, other candidates may back off or run for her seat, which represents all of Hawaii except urban Honolulu.

Hirono is not the strongest of campaigners; her record in Congress is not widely known to many, and she has carried a relatively low profile.

However, some political observers say Hirono has become a better politician and leader since losing a tough campaign to Lingle for governor in 2002, and since serving in Washington for the past four years.

If she receives the backing of U.S. Sen. Daniel K. Inouye and Gov. Neil Abercrombie, Hirono could prove to be a formidable candidate — especially in an election in which Hawaii-born Barack Obama will be on the same ballot.

UPDATE: Inouye’s office released this statement Thursday afternoon: “The Senator will remain neutral until the primary election is finished.”