The deadline for President Obama to secure congressional authorization for the military operation in Libya went whizzing by Friday without such a vote, fueling lawmakers' concerns that the administration was flouting the law, but the White House insisted it was on solid legal footing.

The concerns stem from provisions in the 1973 War Powers Resolution. The resolution, passed in defiance of then-President Nixon at the tail end of the Vietnam War, states that presidents must seek congressional approval to keep U.S. forces in hostilities for more than two months. Friday was the 60th day of U.S. involvement in the U.N.-backed military intervention in Libya.

Asked about the requirements in the law, White House Press Secretary Jay Carney cited the president's ongoing consultation with Congress and claimed his actions "have been and are consistent with the War Powers Resolution." He said the White House would continue to consult with Congress, adding that the administration would "welcome an expression of support" from lawmakers.

But sporadic attempts to cobble together language in support of U.S. intervention so far have not yielded a firm resolution in Congress. The House wasn't even in session this week.

Obama sent a letter to Congress Friday, prodding lawmakers to pass a resolution, while downplaying the scope of U.S. involvement.

"While we are no longer in the lead, U.S. support for the NATO-based coalition remains crucial to assuring the success of international efforts to protect civilians from the actions of the Qaddafi regime," he wrote.

The inaction has raised concern among lawmakers from both parties. Rep. Dennis Kucinich, D-Ohio, told Fox News that a bipartisan House coalition is prepared to move a resolution Monday that would either get Congress to sign off on the intervention or cut off the operation.

Without stating whether or how they might challenge the U.S. military involvement, several other lawmakers called for an explanation from the administration, and fast.

House Armed Services Committee Chairman Buck McKeon, R-Calif., fired off a letter to Obama calling for the president to justify the U.S. military's presence in the country and underscoring the 60-day deadline.

"With America's armed forces in harm's way, it is not my intention to second-guess or undermine your authority as Commander in Chief. However, the War Powers Resolution requires that, within sixty calendar days after a notification is submitted to Congress regarding the introduction of U.S. forces into hostilities, the President terminate any use of U.S. forces unless the Congress has acted. As you are aware, we are approaching this critical juncture," McKeon wrote.

Rep. Justin Amash, R-Mich., in a Detroit News op-ed, called on Congress to assert its authority.

"There has been no better time to regain our Constitutional balance and check the president's war powers. Congress is a co-equal branch -- and it should start acting like it," he wrote, conceding that a related bill he introduced in late March is unlikely to get a vote. He also expressed concern about a defense bill he claimed would grant authorization for use of force worldwide.

Six GOP senators earlier this week penned a letter to Obama asking "whether you intend to comply with the requirements" of the war powers law. A spokeswoman for Sen. Rand Paul, R-Ky., who helped organize the letter, said the senator would wait to see how the president responds before making a decision on how to proceed.

Compliance with the War Powers Resolution has been a muddy area ever since its enactment. Presidents routinely claim it is unconstitutional, but the law has not been struck down in court. Congress and the White House historically have avoided major standoffs over the law because either military intervention has ended before the 60-day deadline or Congress approved the intervention -- for instance, with the two Iraq wars. President Clinton squared off in a War Powers Resolution fight against Congress over bombing in Kosovo in 1999, but the president won that fight.

Sen. John McCain, R-Ariz., who had been working on legislative language in support of the military action in Libya, recently told Fox News that he does not think the law is constitutional either. He said "there doesn't seem to be any great movement towards acting."

But the Obama administration, which cited the War Powers Resolution when it authorized the action in Libya, may be taking precautions.

Several reports have indicated the administration is weighing whether to pause the mission in Libya, only to restart it and presumably reset the 60-day timeline. Or the administration could stop using its drones for attacks.

Kucinich, though, said doing so would merely be a "legal loophole." He suggested the administration would still be in violation of the law "as it was intended."

John Yoo, a former Bush administration attorney who played a key role in drafting its interrogation policies, and Robert Delahunty, a fellow Bush Justice Department official, wrote in a Wall Street Journal column Friday that these tactics would not count as a "fair, honest reading of the War Powers Resolution."