WASHINGTON — Republicans in Congress are beginning the search to find money to help President-elect Donald Trump fulfill one of his biggest campaign promises: to build a wall along the Mexican border to slow illegal immigration.

Trump is expected to ask Congress to provide the initial funding for the project, estimated to cost between $12 billion and $38 billion.

Once construction begins, Trump has said, he will demand reimbursement from Mexico, even though Mexican officials have said they won’t pay.

Trump promised Friday that he would force Mexico to fund the wall.

“The dishonest media does not report that any money spent on building the Great Wall (for sake of speed), will be paid back by Mexico later!” he tweeted.

Former Mexican President Vicente Fox responded: “Trump may ask whoever he wants, but still neither myself nor Mexico are going to pay for his racist monument. Another promise he can’t keep.”

Republicans Friday were trying to figure out how to provide initial support for the wall, despite opposition from Democrats.

“I’m not sure anyone believed the government of Mexico was going to write a giant check to the federal registry, but there are other ways to ensure Mexico does pay for the border buildout,” said Rep. Luke Messer, R-Ind. “There are broad conversations about fulfilling all of Mr. Trump’s campaign promises.”