Trump said in a tweet on Friday: "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!"
But moving forward with that plan would put Republicans and Trump in some very awkward political positions, and it could even lead to a potential government shutdown this spring.
"This would be a huge and ridiculous overreach by Republicans," said nonpartisan budget expert Stan Collender.
Let's break down how funding a border wall could go south real quickly for Republicans in Washington.
They don't have a way to pay for it yet
Trump has estimated his wall would cost $US8 billion ($11.5 billion). The Washington Post's team of nonpartisan fact checkers concluded that's an extraordinarily low number; it would likely cost $US2 billion for just the raw materials. No one has exact figures, but one top construction analyst told our fact checkers the entire wall could cost $US25 billion.
Budget experts say it's not clear where money like that would come from right now.
You could technically take money from already-appropriated border security funds to pay for the wall, but those funds are nowhere near enough: The government is spending an estimated $US4 billion a year on border security. And we should note a 2013 proposal that suggested spending $US46 billion on border security over the next decade failed to pass Congress.
Republicans could raise taxes to pay for it - but that would put them in the awkward position of asking the American taxpayers to contribute more money for a border wall Trump has promised they wouldn't have to pay for.
Which brings us to our second reason this could backfire on Republicans.
It gives Democrats a lot of openings to attack Republicans
If Republicans move forward with trying to fund Trump's wall (or fence or some kind of barrier) with no guarantee of reimbursement from Mexico, they'd be opening themselves up to ridicule by Democrats on several fronts:
1. Broken promises: Democrats could claim Republicans are trying to spend billions of taxpayer dollars on a wall that they promised Mexico would pay for. "They could characterise it as a huge campaign promise break by Trump," Collender said.
2. The deficit: Democrats could claim that now that Republicans are in charge, they're raising the deficit - what the government spends versus takes in every year - with this wall. Another campaign promise broken by Republicans, they could say.
3. Public opinion: Democrats could use the mere fact a wall/fence/barrier is being built to rally their base on immigration. CNN exit polling from the 2016 election shows 54 per cent of Americans oppose building the wall along the entire Mexican border (Trump has said he's willing to skip some parts).
Next would come a game of chicken that Congress has become all too familiar with in recent years. Congress has to pass some kind of spending bill by April 28 so the government doesn't shut down. If this border wall provision is in there, Democrats could use all the arguments above as justification to block it.
One side would have to back down before the April deadline, and budget experts think that will be Republicans, for the reasons I just outlined.
"A shutdown is possible, and I think Republicans would back down almost immediately," Collender predicted.
It could build a wall within the Republican Party
Democrats oppose the wall on principle. But there is a significant number of Republicans who could oppose the wall for its cost. They're known in Washington as deficit hawks; lawmakers whose top priority is to keep spending in line with revenue year after year.
And even without a wall that costs billions of dollars, a Trump presidency is not looking friendly to deficit hawks, says Steve Bell, a GOP budget expert now at the Bipartisan Policy Center.
Bell thinks these hawks will likely hold their noses to pay for a few costly items on Trump's wish list, like a replacement of Obamacare or his infrastructure plan. But add a costly wall on top of all that, and Bell thinks deficit hawks will stop cooperating.
"To now ask them to pay for a wall which we were told Mexico would pay for," Bell said, "within six months, you're going to have a splintering inside the Republican Party in the House and Senate. But also a distancing from Trump."
And a split GOP only increases the threat of a shutdown this spring.
To sum up, a fractured Republican Party, an empowered Democratic Party, and the very real potential for a government shutdown. Those are the risks Republicans undertake in trying to fund Trump's border wall without Mexico's help.
