Hurricane Harvey's multibillion-dollar damage is already setting Donald Trump's plans for his border wall adrift, a political outcome that could help him save face even if it angers his anti-immigration base.

Appropriating recovery funding will let the air out of the U.S. president's threats to shut down the government unless a spending bill allocates seed funding for his long-promised U.S.-Mexico border wall.

But that could buoy him politically as he tries to score a legislative win, says Drexel University disaster historian Scott Knowles.

If Congress prioritizes billions to pass a suitable relief package, giving Trump cover to suspend demands for his wall and avert a government shutdown, "then the storm would have saved his presidency," Knowles says.

White House officials have "quietly notified" Congress that the $1.6 billion Trump demanded for wall funding in a September spending bill could actually wait until a December budget, the Washington Post reported Friday. On Friday Trump sent lawmakers a $7.9 billion request for an initial down payment for Harvey relief and recovery efforts.

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U.S. President Donald Trump participates in a briefing on Harvey relief efforts on Tuesday in Corpus Christi. If Congress prioritizes billions for a relief package, giving Trump cover to suspend demands for a border wall, 'then the storm would have saved his presidency,' says historian Scott Knowles. (Evan Vucci/Associated Press)

Priorities have changed in the wake of the historic storm that cut a path of destruction through Texas and Louisiana, killing dozens and likely doing $200 billion of damage. It further complicates an already crammed September to-do list that awaits Congress.

Members of Congress returning on Tuesday from the summer recess have precious few weeks to pass a budget before the end of the month, raise the debt ceiling, reauthorize the National Flood Insurance Program and take up tax reform. Now with Harvey recovery operations dominating the agenda on Capitol Hill, Trump's estimated $15-billion wall is getting pushed further into the background.

An enormous political opportunity

That's why Knowles sees an enormous political opportunity for a "principled transition" away from a war with Congress for an unpopular border wall that would threaten to trigger an unpopular shutdown of government agencies.

"Trump needs to use the power of his bully pulpit to make it clear there's going to be no government shutdown and Congress is going to get to work right away on a robust relief bill."

Democrats and Republicans would find it difficult to argue against proposed spending for rebuilding the Gulf Coast or new flood-control measures across America.

If Trump frames it as an infrastructure bill — something he promised as part of his agenda — "it would be an enormous political win" for a president in need of one, Knowles says.

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Houses are partially submerged in floodwaters in northwest Houston on Wednesday. (Adrees Latif/Reuters)

The urgency of opening up Harvey aid would at least speed up raising the government's borrowing limit before the federal government runs out of money on Sept. 30.

Congress could ride the post-disaster emotional momentum to attach a debt-limit increase to a spending bill that fiscally conservative Republicans might have otherwise balked at. Avoiding a debt-limit battle would benefit Trump, as defaulting on government debt on his watch could trigger financial calamity.

Former Senate budget committee staff director Steve Bell, who recently believed a government shutdown was "an inevitability," revised his prediction after Harvey made landfall.

Failure to pass a budget by the deadline would force federal agencies to cease all non-essential services. But the first major natural disaster of the Trump presidency has upended the fiscal picture and diminished odds of that happening, says Bell, a senior adviser with the Bipartisan Policy Centre in Washington.

He doubts Trump would dare veto a spending bill containing a down payment on what's likely to be a very expensive rehabilitation effort post-Harvey simply because it omits funds for a wall that 64 per cent of Americans oppose.

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Residents wade with their belongings through floodwaters in northwest Houston on Wednesday. (Adrees Latif/Reuters)

"If he vetoes it because the wall money isn't in there, that would begin to erode his 35 per cent base" of support, Bell said.

"And over something like the wall? I mean, he's choosing between his wall and beginning the relief effort for Harvey. It's kind of a no-brainer. How can he not sign it?"

A stopgap bill?

What's likely to happen instead, Bell predicts, is that Trump will avoid a shutdown by agreeing to put the Harvey funding on a stopgap "90-day continuing resolution" bill to buy more time, keep the government open and let Trump "fight another day" for his wall. That day would come in December.

By then, however, Trump risks having postponed much of his agenda, including tax reform, leaving him with little to show in legislative accomplishments heading into the 2018 midterm elections.

"He's in danger" of too many delays, Bell says. "This guy would sign a piece of toilet paper and hold a signing ceremony if he could get anything out of Congress at this point."

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John Murray attaches a U.S. flag to a sign reading 'Bet They Blame Trump' at his business, damaged in the wake of Harvey, in Rockport, Texas. (Eric Gay/Associated Press)

How Trump works with lawmakers next week will be just part of his response to Harvey.

Although he was faulted for not meeting with flood victims as well for as his campaign-style appearance in Corpus Christi, Texas ("What a crowd, what a turnout!" he addressed hurricane victims), Trump has avoided major missteps, says former Federal Emergency Management Agency director James Lee Witt, who headed FEMA during the Clinton administration.

'I think he's done the right thing'

"I think he's done the right thing by going down to visit an area that wouldn't impact the resources and the flow of recovery," Witt said of Trump's decision to tour Corpus Christi rather than head to major destruction zones in Houston.

Witt also lauded Trump's FEMA pick Brock Long, a widely praised leader who has co-ordinated relief efforts after 14 previous disasters.

It's the president's policies, though, that critics say could undermine how Trump is remembered for his leadership during what's estimated to be the costliest natural disaster in U.S. history.

Two weeks before Harvey ravaged Houston, Trump rescinded an Obama-era executive order to help flood-prone communities rebuild infrastructure to better withstand future disasters.

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Kyle Iselt touches floodwater as his mother Melissa Buchanan and father Shannon Iselt drive a boat in northwest Houston on Wednesday. (Rick Wilking/Reuters)

Harvey made landfall the same year Trump announced the U.S. withdrawal from the Paris pact on climate change, a global weather concern that climate scientist Kevin Trenberth estimates may have contributed 15 per cent of Harvey's rainfall.

Trump also slashed funding for FEMA as well as the National Oceanic and Atmospheric Administration, "the information base that we need to do good warning and forecasts," said Trenberth, who is with the National Center for Atmospheric Research.

Trump pledges $1 million of his own money

At a time when Trump wants to fund a border wall, Trenberth says cuts to weather-research budgets amount to a "penny wise, pound foolish" approach.

"Improved knowledge can translate into 10-fold or even 100-fold benefit in terms of reduced costs down the line," he said.

Trump has pledged $1 million from his own money towards Harvey relief.