A vote in the US House of Representatives that would have forced Donald Trump to immediately end America's war with Iran has ended in a tie.
Three Republicans crossed the aisle to vote with 209 Democrats on a resolution directing the president "to remove the United States Armed Forces from hostilities against the Islamic Republic of Iran".
One Democrat and an independent joined the remaining 210 Republicans to vote no.
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A tied vote in the House means the resolution did not pass.
The three House Republicans who crossed the floor this morning are all facing tight re-election races in November's looming elections.
Earlier this week, Trump lost the support of another Republican in the Senate.
Lisa Murkowski voted against the war for the first time, alongside two other Republican opponents.
Her switch meant legislation to end the war in the upper house failed by two votes.
If the resolution passed both houses, Trump would have no choice but to end the US blockade of the Strait of Hormuz and cease all hostilities with Iran.
The handful of defecting Republicans in a narrowly held Congress should be cause for concern for Trump.
In previous wars, Congress had a habit of rallying around the flag to support it.
The declaration of war on Japan in 1941 passed the House 388 to one, and in the Senate 82 to zero.
The vote was held an hour after President Franklin Roosevelt's "day that will live in infamy" speech.
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But Trump has done very little to convince the American people of the value of the war with Iran.
Democrats are tearing into Trump over the war and its cost.
"It's been 76 days since Trump launched his illegal war in Iran, and the Administration is still refusing to share with Congress or the public an official legal rationale for the conflict—all while pushing for $US1.5 trillion ($A2.08 trillion) for the Pentagon," Senator Tim Kaine said.
"Everything about that is ridiculous."
Congresswoman Pramila Jayapal described the war as "senseless".
"Billions of taxpayer dollars wasted. Thousands of lives lost. Soaring energy prices," she said.
"That's what Republicans just voted for."
While the president is the commander-in-chief of the US military, only Congress has the authority to declare war.
They also have the power to fund the war, which has cost the Pentagon an estimated A$40 billion so far.
Because Congress did not authorise the war, the costs of it are being pulled from the general Pentagon budget.
The Wall Street Journal reported today that the Pentagon will have to cancel training exercises and other spending because so much money is being diverted to the war with Iran.
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