WASHINGTON — President Donald Trump may not need Congress to go to war with Iran.

That's the case his lieutenants have been quietly building as tensions between the two nations have escalated.

The key elements involve drawing links between al Qaeda and Iran and casting Iran as a terrorist threat to the U.S. — which is exactly what administration officials have been doing in recent weeks.

That could give Trump the justification he needs to fight Iran without congressional approval under the still-in-effect 2001 use-of-force resolution.

That prospect is unsettling to most Democrats, and even some Republicans, in part because Iran didn't attack the U.S. on 9/11, in part because there is a reluctance to engage U.S. forces in another theater of war, and in part because many lawmakers believe Congress has given too much of its war-making authority to the president over the years.

With Congress unlikely to grant him new authority to strike Iran under the current circumstances, and amid a campaign of "maximum pressure" against the regime in Tehran that has raised tensions, Trump administration officials have sent strong signals that they will be ready to make an end run around lawmakers, using the 2001 authorization for the use of military force — or "AUMF" in Washington-speak — if necessary.

That law gave the president the power to use force against "nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons."

Earlier this month, the U.S. deployed an aircraft carrier strike group to the region. Three U.S. officials told NBC that a surge in American forces in the region was a response in part to intelligence-gathering suggesting that the Iranian regime had given proxies a green light to attack U.S. personnel and assets in the region.

And in recent weeks, the Trump administration has accused Iran of assisting al Qaeda, designated an arm of the Iranian military as a foreign terrorist organization and accused Iran of being linked to a terrorist threat against the U.S. embassy in Baghdad, Iraq.

National Security Council officials declined to speak on the record with NBC about whether such incidents would satisfy the legal threshold necessary for the president to determine he had the authority to use force against Iran.

But former government lawyers familiar with the 2001 law and its applications say it's obvious from those moves what the Trump administration is trying to do.

"The whole thing is building up to the notion that they don't have to go to Congress for approval," Yale University law professor Harold Koh, who served as the State Department's top lawyer under Secretary Hillary Clinton, said in a telephone interview with NBC News.

Yet Koh said an attempt to shoehorn Iran into the 2001 AUMF is absurd and shouldn't pass legal muster.

"The theory of war powers has to be that Congress doesn't just sign off once," he said. "The suggestion now that Iran attacked us on 9/11 is ridiculous."

The original law essentially creates a two-part test for the president to make a determination that force is warranted: A country, group or person has aided al Qaeda and force is necessary to prevent a terrorist attack against the U.S. from that entity.

Under questioning from Sen. Rand Paul, R-Ky., a critic of the executive branch's expansive view of its war powers under both Presidents Barack Obama and Trump, Secretary of State Mike Pompeo said last month that he would "leave it to the lawyers" to sort out whether Trump had the authority to go to war with Iran absent a new authorization from Congress.

But he also forwarded an argument that he has been making since the early days of the administration that is tantamount to a case that the first part of the test has been met.

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"The factual question with respect to Iran's connections to al Qaeda is very real. They have hosted al Qaeda, they have permitted al Qaeda to transit their country,” he said at a Foreign Relations Committee hearing. “There is no doubt there is a connection between the Islamic Republic of Iran and al Qaeda. Period, full stop.”

There has been intense debate in recent years about the extent to which the remnants of al Qaeda have found assistance in Iran, with Iran hawks taking the position that the ties are deep and significant and others contending that attempts to link the Shia regime to terrorism carried out by Sunni groups are wrong or disingenuous.

But the deployment of more forces to the region to counter the threat of attacks on American personnel and assets, as well as the partial evacuation of the U.S. embassy in Baghdad, could be seen as satisfying the second part of the use-of-force test. That is, the idea that force is appropriate to prevent a terrorist threat from a country that has given assistance to al Qaeda.

House Speaker Nancy Pelosi, D-Calif., said last week that she appreciates that Trump has generally been reluctant to go to war and cast his advisers as the drivers of the current escalation of tensions. She said the president doesn't currently have the power to go to war with Iran.

“The responsibility in the Congress is for Congress to declare war,” she said. “So I hope the president’s advisors recognize that they have no authorization to go forward in any way. They cannot call the authorization, AUMF, the authorization for the use of military force, that was passed in 2001, as any authorization to go forward in the Middle East now,”

Trump himself has left the door open.

Asked about the possibility this week, he said, "I hope not."

But there's little question that his administration is getting ready — and getting ready to go it alone.