Mexico won't pay for it. Democrats hate it. Border-state Republicans don't like it. Congressional GOP leaders would rather not undertake it. There could be an avoidable government shutdown over it.

And yet President Trump's budget director is pushing Congress to spend $1.4 billion to start building a wall along the U.S.-Mexico border.

The political tension could backfire on the president. Trump's insistence on a wall is increasingly doing what some had warned it would do: It's undermining his relationship with Congress, it's putting Republican leaders in a no-win scenario on whether to fund it, and it's potentially hindering the president's ability to push other issues and policies.

Who is really going to pay for Trump's border wall? (Peter Stevenson/The Washington Post)

Here's why:

1. It's building a wall between Trump and his party

“Building a wall is the most expensive and least effective way to secure the border.”

That's Rep. Will Hurd (R-Tex.), whose district spans about 40 percent of the entire southern border, in a statement in January.

Hurd still doesn't support it, nor, according to a recent survey of border-state lawmakers by the Wall Street Journal, do any lawmakers in Congress who represent constituents on the border. That includes the Senate's No. 2 Republican, John Cornyn of Texas.

Trump says his wall would “secure, protect and defend” Americans. But right now, he can't even persuade Republican lawmakers whose constituents would ostensibly benefit the most from it.

Pushing wary lawmakers to embrace the wall risks further straining Trump's relationship with his party, both with border lawmakers and with GOP leaders trying to avoid a shutdown. Speaking of …

2. It's threatening a government shutdown

Congress is facing a deadline of midnight Friday to pass a spending bill to keep the government open. Both Democratic and Republican leaders say they are successfully navigating the sinkholes that come with such a spending debate. (Similar political dynamics shut down the government in 2013 and very nearly in 2015.)

Funding Trump's border wall has not been part of that bipartisan plan. It's just too risky a debate to undertake when a government shutdown is on the line: Conservative Republicans are wary of the untold billions the wall would cost, while border-state lawmakers and basically all Democrats oppose it. That's more than enough opposition to kill any spending bill.

But the Trump administration has a different perspective. April 29 marks the president's 100th day in office, and he badly needs a win. He's looking to score one by getting funding for his centerpiece campaign promise. “We want wall funding,” Trump's budget director, Mick Mulvaney, told the Associated Press on Thursday.

It's not clear how Republican leaders can appease their president by funding the wall while avoiding a shutdown. And that impasse is a big reason budget experts say there's a 50-50 chance the government shuts down next week.

3. It's uniting Democrats

Democrats have their own divisions to deal with. But opposing Trump's wall is a near-perfect rallying cry almost everyone in their party can get behind. It's just too good an opportunity to whack Trump and Republicans in Congress.

If Congress funds Trump's wall, Democrats can argue that the president has broken yet another campaign promise by building a wall without getting Mexico to pay for it. (Trump says he eventually will force Mexico's hand.) They can also argue that Republicans are raising the deficit and that they're teetering on the brink of a shutdown when they control Washington because of this wall.

Perhaps most important, Democrats have public opinion on their side.

While popular with his base, Trump's wall has never been that popular with the rest of the United States. Fifty-four percent of Americans oppose building a wall along the entire Mexican border, according to CNN exit polling from the 2016 election. (Trump has said he's willing to skip some parts.)

A recent Texas Lyceum poll found that in Texas (Trump country), 61 percent oppose his wall.

4. It's blocking Trump's ability to get other things done

Trump's relationship with Congress right now isn't great.

Democrats despise him. Republicans want to work with him, but his leverage with the party is questionable. (Witness Republicans' inability a few weeks ago to pass a health-care bill despite Trump's urging, and growing evidence in places like Georgia that Trump's political sway won't make or break elections.)

Trump can't afford to exacerbate tensions with either side of Congress if he wants to overhaul the tax code or restart health care, or — siren alert — avoid another government shutdown in October, when Congress has to pass a spending bill for 2018.

For the president, pushing his centerpiece campaign promise right now is undermining the rest of his agenda by alienating the very people he needs to pass laws.