Several military justice officials voiced concerns at a House of Representatives hearing that the proposed legislation would deny key rights to the "war on terror" detainees normally accorded defendants under US civilian and military law.
Human rights groups said the commissions under the White House's plans would violate basic standards of fair justice.
"The administration is asking Congress to rubber stamp a law that would undermine some of our most basic criminal justice rights," said Jennifer Daskal of Human Rights Watch.
David Hicks
Mr Bush has urged the US legislature to pass a bill that would permit almost the same type of military tribunals for detainees at the US Navy prison in Guantanamo, Cuba, as those that were halted when the Supreme Court in June.
Among those who could face such a tribunal is Australian David Hicks, who is currently being detained in Guantanamo Bay and awaiting an outcome to his case.
According to Mr Bush's proposal, suspects could possibly end up being tried and sentenced to death by the military tribunals based on "hearsay" evidence and testimony produced by coercion, which is not allowed in normal US military justice proceedings.
Defendants could be kept from the proceedings when the court hears secret evidence, and so would not be able to respond to it.
Moreover, Guantanamo prisoners could be tried for "conspiracy" even though international law has held since the post-World War II Nuremburg trials that conspiracy cannot stand alone as a war crime.
Of about 450 prisoners at Guantanamo, 10 have been charged so far, seven of them solely for conspiracy.
Also under the White House's proposed rules, if convicted the prisoners can appeal to a military court, but have no right, unlike current US law, to contest their conviction in a federal court or the Supreme Court.
Such a framework would effectively annul all of the recent challenges in civil courts made by Guantanamo prisoners against their detentions.
Defence Department lawyers
Top US Department of Defence legal officials expressed reservations about a number of these stipulations in the proposed bill.
"I would recommend that the committee look at not allowing coerced statements," Rear Admiral Bruce MacDonald, the Navy's judge advocate general, told the hearing.
"Statements that are obtained under torture are excluded under the current commission rules," he said, unless the statements "meet a reliability and probative test."
The US Marine Corp's judge advocate general, Brigadier General James Walker, said he was "concerned about any process which would permit the introduction of evidence against an accused outside of his presence," a position echoed by his army counterpart Major General Scott Black.
Representative Victor Snyder hit out at the legislation in the hearing, asking how the military officials would feel is the White House's proposed version of military tribunals was used to try Americans.
"Would you all want to be tried if you were captured by somebody, with a piece of classified information that you could not confront?" he asked.
Resistance was already mounting Thursday to Bush's request for the powers to set the military tribunals.
Three senators of Mr Bush's own Republican Party, including possible 2008 presidential candidate John McCain, have moved together with opposition Democrats to moderate the White House proposal.
Mr Bush has already given some ground to critics: the proposed law will require the unanimous agreement of a dozen military judges to impose the death penalty.
It also no longer asks that an acquitted "enemy combatant" can still be held until the official end of hostilities.
But the American Civil Liberties Union said that Mr Bush's bill remains highly questionable under US laws.
"The new proposal has nearly all of the same problems (as the tribunals blocked by the Supreme Court), and will eventually be found illegal," the civil rights organisation said in a statement.
Human Rights Watch said in a statement that the White House bill "violates almost all the basic norms of fair justice highlighted by the Supreme Court" in its June ruling.