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WASHINGTON — Sen. Tim Kaine, D-Va., is demanding the release of a secret memo outlining President Donald Trump’s interpretation of his legal authority to wage war.

Kaine, a member of the Armed Services and the Foreign Relations committees, sent a letter Thursday night to Secretary of State Rex Tillerson seeking a seven-page memo the administration has kept under wraps for months.

Kaine has been leading the charge for Trump to outline his legal rationale for a U.S. bombing campaign in Syria last April in response to President Bashar al-Assad’s chemical attacks on civilians. Kaine and others worry that such action compromises congressional oversight over military action.

There is a new urgency to obtain the memo given increasing U.S. involvement in Syria and recent Trump administration rhetoric on North Korea. Shortly after the 2017 bombing raid, several members of Congress called on Trump to justify it under U.S. and international law. Article I of the U.S. Constitution gives Congress the power to declare war.

“The fact that there is a lengthy memo with a more detailed legal justification that has not been shared with Congress, or the American public, is unacceptable,” Kaine said in the letter to Tillerson, obtained by NBC News.

It has been standard for U.S. presidents to release their legal arguments behind military strikes dating to the Korean War, according to Protect Democracy, a bipartisan group of lawyers. There have been a few exceptions, including the U.S. invasion of Grenada in 1983 under President Ronald Reagan.

“I am also concerned that this legal justification may now become precedent for additional executive unilateral military action, including this week’s U.S. airstrikes in Syria against pro-Assad forces or even an extremely risky ‘bloody nose’ strike against North Korea,” Kaine wrote.

Related: U.S. strikes pro-Assad forces in Syria after SDF is attacked

According to a court filing provided by Protect Democracy, Attorney General Jeff Sessions was briefed last April on the substance of the memo. Sessions received the briefing so he could know “how to advise the president on future actions,” the filing said, citing a Department of Justice attorney.

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The Justice Department declined a request for comment.

Kaine’s demand comes after a U.S.-led coalition fighting ISIS in Syria conducted air and artillery strikes against pro-regime forces on Wednesday, killing an estimated 100 pro-regime fighters. The confrontation and last year’s operation “raises serious questions about our continued presence in Syria,” Kaine said.

It also follows recent talk from Trump administration officials — including United Nations ambassador Nikki Haley — suggesting the U.S. stands ready to respond militarily to additional chemical attacks in Syria.

Lawmakers are also concerned about the administration’s adversarial posture toward North Korea, including reported internal discussions of a first strike. On Monday, a group of Democratic senators organized by Sen. Martin Heinrich of New Mexico, sent a letter to Trump saying he lacks the “legal authority” to carry out a pre-emptive strike on North Korea.

Trump has previously suggested on Twitter he may dispense with diplomacy in North Korea, contradicting his own officials, including Tillerson.

North Korean Leader Kim Jong Un just stated that the “Nuclear Button is on his desk at all times.” Will someone from his depleted and food starved regime please inform him that I too have a Nuclear Button, but it is a much bigger & more powerful one than his, and my Button works! — Donald J. Trump (@realDonaldTrump) January 3, 2018

Meanwhile, his administration has not committed to seeking prior approval from Congress or the United Nations Security Council for any potential action against North Korea.

Finally, Trump’s demand for a U.S. military parade in Washington, an event that has historically been associated with major wars, has raised concerns on both sides of the aisle.

“Unless Donald Trump goes to Congress before starting a new war, the real bloody nose is going to be the American Constitution,” said Allison Murphy, counsel at Protect Democracy who served in President Barack Obama’s White House Counsel’s office.

“Congress needs to demand the secret Syria memo when the administration is threatening to use force around the world without authority,” Murphy said, adding that the American people deserve to see it.

The memo’s existence came to light last fall because of a Freedom of Information Act request by Protect Democracy seeking Trump’s legal justification for the strikes.

Since the military action in Syria was against a foreign sovereign country — and not ISIS — there was no obvious legal basis for it, the group argued. In July, a U.S. District Court judge ordered federal agencies to expedite their responses to the group’s FOIA request.

Protect Democracy has also filed a lawsuit to determine whether the Trump administration is developing any analysis about a legal basis for a potential pre-emptive attack on North Korea.

The administration has shared a summary explanation of the memo, which Kaine read aloud during a Dec. 13 Senate hearing, claiming it “would completely wipe out Congress’s power (to declare war) under Article I.”

Defense Secretary James Mattis suggested last week that the U.S. could launch further attacks against Syria. “You’ve all seen how we reacted to that,” he said, referring to the April strikes.

A few days later, Haley warned the U.S. will “not give up on the responsibility” to “provide real accountability for chemical weapons use in Syria.”

Kaine’s bid for more disclosure is part of a broader controversy over how legislation passed shortly after the Sept. 11, 2001, attacks is being used for an open-ended battle against Islamic terrorist groups, including ISIS, that are not covered under the current version of what’s called an AUMF, or authorization to use military force.

Talks on Capitol Hill to approve a new AUMF have stalled amid disputes over issues including how to limit the war’s geographic reach. Some lawmakers are also pushing back, leery of restricting the executive branch’s interpretation of his current wartime powers.

Along with Sen. Jeff Flake, a retiring Republican from Arizona, Kaine has proposed a new war authorization bill. Others who’ve been outspoken include Sen. Bob Corker, a retiring Tennessee Republican who said in October that Trump’s reckless threats against other countries could put the U.S. “on the path to World War III.”