(CNN) The Pentagon is staring down weeks and possibly months of budget uncertainty that's shaping up to be the most turbulent budget drama for the military in years.

Once again, the US Armed Forces are caught in the middle of a bigger political fight -- this time over President Donald Trump's border wall -- where the defense budget could become little more than a pawn in spending negotiations, defense analysts and congressional aides say.

At a rally in Phoenix on Tuesday , Trump threatened to shut down the government if Democrats would not fund his proposed wall on the US-Mexico border, a move that Republican leaders said they disagreed with.

The wall presents yet another potential stumbling block for Congress to pass spending bills for the military and the rest of the federal government this fall, in addition to the challenge of raising the debt ceiling

But even without a protracted fight over the wall or debt limit, military funding could swing by a whopping $70 billion depending on whether congressional Republicans get their way on the defense budget.

"The future is as murky as it has ever been," said Fred Bartels, a defense analyst at the Heritage Foundation. "When they went into recess, basically no one knew where to go, and I don't think anything has changed during August."

At issue is the huge gap between the spending caps that limit Pentagon spending under the 2011 Budget Control Act and the defense budgets proposed by the Trump administration and congressional Republicans.

The Trump administration proposed $603 billion for base national security spending, and the House Republican budget plan would authorize $622 billion. But the 2011 law limits national security spending to $549 billion, unless Congress lifts the caps.

Congress has raised the budget caps in the past thanks to short-term bipartisan budget deals, but doing so may be more difficult in the current political climate, which is why talk of a possible government shutdown has already begun.

"At this point the range of uncertainty is really high," said Todd Harrison, a defense analyst at the Center for Strategic and International studies. "The lowest it could possibly be is the budget caps. That's the default if all else fails. I've not heard anything on what a broader budget deal looks like. It remains a mystery."

In the near-term, a stopgap spending measure called a continuing resolution is likely to avoid a government shutdown on October 1, which keeps government spending at the same level as the current year.

But a continuing resolution comes with its own problems for the military, primarily that the stopgap spending measure doesn't allow the military to start new programs.

Defense Secretary James Mattis warned Congress against going that route earlier this month, saying that it would prevent the military from adapting on issues like electronic warfare, space, cybersecurity and countering enemy drones.

"It just creates unpredictability. It makes us rigid," Mattis said. "We cannot deal with new and revealing threats. We know our enemies are not standing still. ... So, it's about as unwise as can be."

It's normal over the past decade for Congress to use a continuing resolution, or CR, at the start of a fiscal year before passing a full appropriations bill.

Susanna Blume, a former Pentagon official and defense fellow at the Center for a New American Security, said the stopgap spending measure becomes more of an issue the longer the year goes on because the Pentagon can't shift money between accounts like procurement and maintenance.

"The misalignment of the color of money is a serious problem under a long-term CR," Blume said.

For the 2017 budget, the Pentagon was stuck under a continuing resolution for seven months until Congress passed a budget deal in April.

But the issue is even starker for the military this year because of the wide gap of funding — leaving open the question about whether new ships will be built or thousands of new recruits can join as part of Trump and Republican defense hawks' desired military buildup.

A House GOP aide said that defense hawks were likely to press harder than they had in the past to limit the length of a CR.

"Every month you eat into the new fiscal year is potential rebuilding of readiness that's getting deferred, at a time when the problems with readiness are becoming more and more apparent," the aide said. "I think you will find members pretty animated."

The path ahead to a spending deal, however, is treacherous.

While the House passed a spending package last month that included $658 billion for the Pentagon -- $584 billion in base spending and $74 billion in war funding that's not subject to the budget caps -- the Senate has not passed any appropriations bills this year.

Many Democrats have acknowledged the need for more defense spending, but they are demanding equal increases to funding for domestic agencies, too.

That was the basis for previous budget deals under Obama, but with Republicans controlling both the Legislative and Executive Branches, many Republicans do not want to strike a similar agreement this year. Trump's budget proposal cut $54 billion from domestic spending, the same amount it proposes boosting defense.

Congressional aides say there have been little talks between the two parties and the White House about a budget agreement so far, with more energy being focused currently on the fight over funding Trump's border wall and also raising the debt ceiling.

Mackenzie Eaglen, a defense analyst at the American Enterprise Institute, said there's a chance that no budget deal comes together, and Congress adds as much spending as it can above the caps through the war budget, known as Overseas Contingency Operations, in order to skirt the budget caps.

Under that scenario, however, the increase is likely to be smaller than what the Trump administration and congressional Republicans are seeking.

"If that is the vehicle used to increase defense, the best case is then a $15 or 20 billion bump from the caps," Eaglen said.