President Donald Trump promised swift emergency funding to help Texas recover from Hurricane Harvey, but Republicans in Congress will have the ultimate say over how much aid is delivered -- and how quickly it begins to flow.

Trump may have to wait a while. After previous storms, lawmakers have usually demanded detailed spending plans for emergency funds, while conservatives have argued that disaster funding should be offset by domestic spending cuts.

With Harvey still expected to dump rain over Texas and Louisiana for several days, the full scope of the damage isn’t yet known.

Trump said he has already spoken with members of Congress, but his administration hasn’t submitted a formal spending request for additional relief funds.

“The real number, which will be many billions of dollars, will go through Congress,” Trump said Monday at a news conference ahead of a planned visit Tuesday to Texas to meet with local officials involved in the response. “It will happen very quickly.”

‘Robust Measure’

As the scale of damage from the storm became apparent, Texas Democrat Sheila Jackson Lee said she will draft a "robust" relief measure to be added to a spending bill on the House floor next week.

Republican leaders, however, haven’t committed to a swift debate. A spokeswoman for House Speaker Paul Ryan of Wisconsin, AshLee Strong, said Congress "will help those affected by this terrible disaster."

"The first step in that process is a formal request for resources from the administration,” she said.

Congress returns Sept. 5 from its August recess, when it will face a pileup of urgent tasks, including raising the nation’s debt ceiling and passing a stopgap measure to fund the government after Sept. 30. The House is only scheduled to be in session for 12 legislative days in September, which compounds its challenge.

The stopgap bill could be a natural place to add in disaster relief funding, even if it’s just a down payment ahead of a more comprehensive relief measure later in the year. That’s how Congress handled Hurricane Katrina in 2005, when it approved $10 billion in immediate aid while most members were still on August recess. It followed up months later with a $51 billion package.

Border Wall Fight

Trump, however, has threatened to shut down the government over the stopgap spending measure if it doesn’t include funding for his proposed border wall with Mexico.

On Monday, Trump was asked if his shutdown threat could interfere with efforts to assist Texas. "I think it has nothing to do with it really,” he said. "I think this is separate."

Another potential obstacle is demands by House conservatives to cut spending to pay for a response to Harvey.

"It’s a little early to say, but I have no doubt in the size of the federal budget we have we can find lower priority spending to help the people of the Gulf Coast who have been adversely impacted by this tragedy," said Jeb Hensarling, a Texas Republican and chairman of the Financial Services Committee.

Even so, a number of lawmakers in both parties have called for a bill that would be exempt from budget caps requiring offsetting spending cuts.

“We will need to put together an emergency supplemental appropriations bill,” Republican John Culberson, a senior member of the House spending panel who represents Houston, said Monday in a Bloomberg Television interview. “No one could have ever predicted or expected a catastrophe of this magnitude to descend on the Houston area.”

‘Timely Relief Bill’

House Minority Leader Nancy Pelosi of California issued a statement Monday saying, “Republicans must be ready to join Democrats in passing a timely relief bill that makes all necessary resources available through emergency spending.”

So far, the Trump administration hasn’t determined whether additional funds will be needed, acting Homeland Security Secretary Elaine Duke said Monday on Bloomberg Television. She said if additional funds are needed, the Department of Homeland Security will work with Congress to obtain them.

Trump is taking pains to show himself actively engaged in the federal response to Hurricane Harvey as he faces the first major natural disaster of his presidency with his popularity at an ebb. He said he has been in frequent contact with the Republican governor of Texas, Greg Abbott.

The president and First Lady Melania Trump were flying to Texas on Tuesday even as Harvey, now a tropical storm, continues to deluge the region. An airspace restriction issued by aviation authorities suggests Air Force One will land in Corpus Christi, away from the worst ongoing flooding in the Houston region.

A House Republican aide said that the Federal Emergency Management Agency has sufficient disaster relief funding for now. FEMA said it has $3.3 billion in the disaster relief fund as of Monday, which includes money that can be shifted from longer-term priorities. Costs associated with Harvey are “quickly drawing down the remaining balance” in the fund, said Stephanie Moffett, a FEMA spokeswoman.

A deluge of rain and rising floodwaters has left Houston immersed. Harvey, which made landfall as a category 4 hurricane, has drifted back toward the Gulf of Mexico and is poised to regain strength before crashing ashore again, this time on the Texas-Louisiana border.

Harvey’s cost could reach $42 billion, Chuck Watson, a disaster modeler with Enki Research, said in a note to clients Tuesday. That’s up from his projection Monday of a $30 billion impact on the labor force, power grid, transportation and other elements that support the region’s energy sector. Analysts including David Havens at Imperial Capital have estimated the costs could top $100 billion, potentially causing more havoc than Hurricane Katrina in 2005, which spurred at least $118 billion of losses.

Vice President Mike Pence said that as many as a half-a-million people may be ultimately eligible for assistance.

In the past, the Republican Party has been divided over whether disaster spending needs to be offset by other domestic spending cuts, a debate that could return over Harvey response funding.

Both of the Republican senators representing Texas, John Cornyn and Ted Cruz, voted against a $50 billion Superstorm Sandy spending bill in early 2013 after pushing for spending cuts to be attached. Cornyn and Cruz say they voted "no" in the end because of unrelated spending attached to the package.

Representative Peter King, a New York Republican, lashed out at Cruz on Twitter Saturday over his Sandy vote.

"Ted Cruz & Texas cohorts voted vs NY/NJ aid after Sandy but I’ll vote 4 Harvey aid. NY wont abandon Texas. 1 bad turn doesn’t deserve another," King tweeted.

Brian Babin, a Republican whose southeast Texas district has experienced significant flooding from Harvey, said the area will “absolutely” need federal help for the recovery effort.

“This is going to be one of the most expensive, devastating storms in history -- if it’s not the most -- when it’s all said and done,” Babin said in an interview. “This is certainly not a time to start dividing politically, certainly not a time to start adding things that aren’t really connected to the storm.”