Sunday night was just the most recent example this year of President Obama asking Congress to authorize his use of military force to fight the Islamic State.

"If Congress believes, as I do, that we are at war with ISIL," he said, using another acronym for the Islamic State or ISIS, "it should go ahead and vote to authorize the continued use of military force against these terrorists."

Obama has been calling for Congress to get behind his limited military action in Iraq and Syria for more than a year now; he even sent a draft of an authorization of use of military force — or AUMF in D.C. parlance — over to Congress in February.

There are some lawmakers on both sides of the aisle who agree with Obama that Congress needs to vote one way or the other. It's the legislative branch's constitutional duty, they say.

But the president's draft is collecting dust on Capitol Hill, and his pleas are falling on party leaders whose minds are already made up not to act. That's because lawmakers have little to gain but plenty to lose by voting on whether to authorize military force that is, oh by the way, already underway and progressing with or without their say-so.

Here are three big reasons why:

1. Both sides have something to dislike about it

Should the U.S. only focus on taking out Islamic State? What about Syrian President Bashar al-Assad? Is what Obama wants too expansive or too narrow?

"There's no agreement on the fundamentals," said Phil Carter, the director of the Military, Veterans, and Society Program for the nonprofit, nonpartisan national security think tank, the Center for a New American Security.

Bringing up an AUMF for a vote would most certainly invite a messy debate about expiration dates, boots on the ground, drones, the legacy of the Iraq War — all without the guarantee anything would get passed.

And both parties have different reasons for wanting to avoid such drama, said Sarah Binder, a congressional expert with the Brookings Institution.

On the Democratic side, it's just not popular to reengage in military action. "The Democratic base is unlikely to support a return to the Bush-era wars in the Middle East," Binder said in an email. Hillary Clinton is perhaps the best example of why not: The 2016 Democratic front-runner has spent more than a decade defending her vote to authorize military force in Iraq. That vote arguably cost her the 2008 Democratic primary. "It was a mistake," she now regularly says.

On the Republican side, there are hawks who think most of the AUMF proposals have been too limited in scope. Passing one that limits presidential authority in time or scope would essentially be telegraphing America's plans to the enemy, they argue.

In other words, even as Congress seems to be okay with Obama's actual use of force, coming up with a specific use of force resolution that assuages concerns of and satisfies both sides would be a difficult trick.

2. Four numbers: 2-0-1-6

Who wins the White House in 11 months is very much an open-ended question, and that's a disincentive for both parties to avoid this whole AUMF debate.

For Democrats, what good is handing over authority to engage in Iraq and Syria if the next president is a Republican?

For Republicans, what good is it giving Obama wide latitude to fight terrorism when their presidential candidates are on the campaign trail every day criticizing the president's effectiveness?

With just 40 percent of the public approving of Obama's handling of terrorism of late, according to a recent Washington Post-ABC News poll taken in the days after the attacks in Paris, Republicans have a solid political argument to make for why they should be in the White House. A Republican Congress voting to expand Obama's powers to fight the Islamic State might deflate that and/or change the course of an issue that isn't helping his party.

"If Republicans believe that they can win the White House by being tougher on national security issues than Democrats," Binder said, "why vote to empower a Democratic president?"

Another idea could be to pass an AUMF that has the broadest authority possible, Carter said. But ghosts of past blank checks — the Gulf of Tonkin during the Vietnam War or even the most recent Iraq War — would warn that might be a bad idea, Carter said.

And as the threat of the Islamic State constantly evolves, it can be tough to predict what powers a president might need. Will we need to engage on the home front? Expand outside Iraq and Syria? Will we need to have this debate all over again and get a new AUMF in a year or two?

There are just too many uncertainties for a Congress that doesn't cope well with them.

3. There's no immediacy

It'd be nice, Obama has said — and repeated Sunday from his office — to have a renewed AUMF. But it's also not necessary.

Obama and his cabinet officials have argued they already have the authority to conduct air strikes and send in special forces in non-combat roles based on a 2001 AUMF enacted in the days after the Sept. 11, 2001, terrorist attacks.

Having a new one wouldn't change their plans, the Obama administration has said. Sure, passing an AUMF would put them on a marginally better legal and political footing in the future, Carter said. But he figures the real reason the administration wants it is to send a message to the world that the U.S. is united in defeating the Islamic State.

"Wars are a contest of will," he said, "and a clear statement of national will that reflects national consensus can have a powerful effect on distant battlefields." 

That's hardly a bad thing. But the problem with that argument is that Congress acts best — or, one could argue, acts at all — when it's under pressure. It's how the 2001 AUMF and another authorization of force in Iraq a year later both got passed; President George W. Bush argued engaging abroad were matters of imminent national security.

This time around, a president arguing 'it'd be nice, but isn't necessary' isn't exactly the motivator Congress needs to take a vote on a politically troublesome and uncertain authorization of use of military force.