WASHINGTON (Reuters) - As NATO leaders gather in Washington for their 75th anniversary summit this week, a dozen U.S. senators asked them on Wednesday to plug what they see as a Hawaii-sized hole in the North Atlantic Treaty, the military alliance's founding document.

Written a decade before Hawaii became a state in 1959, the treaty's Article 5 - which commits all members to collective self-defense - only covers territories north of the Tropic of Cancer.

The Hawaiian archipelago, home to 1.44 million Americans, is south of the Tropic of Cancer. Members of Congress have been pushing for years to resolve its status, often noting that the December 1941 Japanese attack on Pearl Harbor triggered the U.S. entry into World War Two.

In a letter on Wednesday to Secretary of State Antony Blinken, seen by Reuters, a dozen Democratic and Republican U.S. senators reminded Blinken of "the importance of clarifying that members of the North Atlantic Treaty Organization (NATO) would consider an armed attack against the State of Hawaii to be an attack against all NATO countries."

The letter notes that, when the Senate Foreign Relations Committee recommended ratification of the NATO treaty in 1949, it was with the understanding that overseas territories would not be covered.

"At the time, Hawaii was a U.S. territory and the drafters of the Treaty were reluctant to include all territories of the Treaty Parties under the NATO security umbrella. However, the world has changed significantly since 1949," said the letter, signed by both of Hawaii's senators Democrats Brian Schatz and Mazie Hirono and 10 other senators, including six Republicans.

The Indo-Pacific has become an increasingly important focus of U.S. national security strategy, with Washington viewing China as the main rival to the United States, and monitoring threats from North Korea.

"Silence on whether NATO allies would come to the defense of Hawaii undermines our strategy of deterring conflict in the Indo-Pacific," the letter said, calling for the North Atlantic Treaty to be formally amended.

The letter - also copied to Secretary of Defense Lloyd Austin - set out a series of questions for Blinken, including whether the State Department had sought to amend the treaty to include Hawaii.

"The scars of the attack on Pearl Harbor are still visible today," it said.

(Reporting by Patricia Zengerle; Editing by Chizu Nomiyama)

By Patricia Zengerle