President Trump took a further step back this week from his pledge to force Mexico to pay for the border wall, instead saying he is considering sticking solar panels on top of the fence and selling the energy to help fund the costs.

The proposal — which he said he came up with on his own and was revealing for the first time — is the latest in a long line of suggestions for covering a price tag that could easily top $20 billion.

“We’re talking about the southern border, lots of sun, lots of heat. We’re thinking about building the wall as a solar wall so it creates energy and pays for itself,” the president told supporters at a rally in Iowa on Wednesday night. “And this way, Mexico will have to pay much less money. And that’s good, right?”

Analysts said it’s doubtful that solar energy could pay for the entire cost of the wall but could provide a decent sum — the same as many of the other options suggested on Capitol Hill and across the country.

At least three plans have been introduced in Congress.

Sen. Ted Cruz, Texas Republican, has written the El Chapo Act, which would take money forfeited by drug kingpins such as the infamous former head of the Sinaloa drug cartel, and earmark it for wall construction. Mr. Cruz said the U.S. government is seeking some $14 billion in forfeited drug proceeds from El Chapo alone.

Sen. Luther Strange, Alabama Republican, and Sen. David Perdue, Georgia Republican, joined up on a bill to withhold federal transportation money from sanctuary cities. They said it would serve the dual purpose of finding money for building a wall and encouraging more cities to cooperate with federal deportation agents.

“This legislation will restore the rule of law in sanctuary cities while helping fund the president’s promise,” Mr. Strange said.

His proposal has earned the backing of Sen. Ron Johnson, Wisconsin Republican and chairman of the Senate Homeland Security Committee.

In the House, Rep. Mike D. Rogers, Alabama Republican, has written the Border Wall Funding Act, which would impose a 2 percent fee on remittance money wired from the U.S. to Mexico and dozens of other Latin American countries. Mexico alone got some $27 billion in remittances last year, so a 2 percent fee could have earned Uncle Sam more than a half-billion dollars.

Mr. Trump had floated a remittances tax during the presidential campaign, and there is precedent for it. Oklahoma imposes a tax on funds wired outside its borders.

But the president now appears to be going in a different direction with his solar-panel plans.

It’s an idea Vijay Duggal, an architect in New York, suggested earlier this year. He said he was thrilled when he heard Mr. Trump is on board.

“I’m very excited. Hopefully he follows through,” he told The Washington Times.

He submitted plans to the Homeland Security Department informally but didn’t take part in the ongoing competition for vendors trying to win the wall contract. Mr. Duggal said he is an architect, not a full-service construction contractor.

His concept, which he posted months ago to Border-Wall.us, calls for a 50-foot wall with five tiers of solar panels. But he also includes wind turbines, saying solar energy alone wouldn’t be enough to pay for the wall. He calculated revenue of $1.2 billion a year from both sources.

Mr. Duggal said there are ways to make the wall rugged enough to withstand the rigors of the border, which include repeated attempts to cut or smash through existing walls. Mexicans also regularly throw rocks at agents on the U.S. side of the wall. Mr. Duggal said pre-cast concrete panels behind the solar panels could prevent that.

Perhaps the most unique proposal came earlier this year from Sheriff Thomas Hodgson in Bristol County, Massachusetts. He suggested that jail inmates could volunteer to help build the wall as part of prison work programs.

The proposal proved to be controversial. The Massachusetts House last month passed legislation that would ban the proposal. That bill is now awaiting action in the state’s Senate.

Officials at U.S. Customs and Border Protection declined to comment Thursday on Mr. Trump’s suggestions.

They have already selected a group of contractors who submitted plans this year. Those plans are being refined, and a group of finalists will be paid to build prototypes in California this year.

CBP will then subject the prototypes to a series of tests. According to the contract documents, CBP was hoping for a 30-foot fence that would be imposing in appearance, would deter climbers and would take four hours to puncture.

Mr. Trump originally had said he wanted a wall across the southwestern border, but his top aides have since backed off that. The president also said during the campaign that Mexico would pay for the cost, though the first several budget submissions shunt the cost onto American taxpayers.

The president had said he would try to soak Mexico for the money later, but his comments Wednesday about reducing the price tag for Mexico signal he may have even given up on that hope.

Some 354 miles of the border are already fenced, and another 300 miles have vehicle barriers that are meant to stop cars and trucks barreling through but allow pedestrians easy access. In his budget, Mr. Trump asks for 32 more miles of fencing to be built in Texas at a cost of $784 million, or $24.5 million a mile.

That is four times the $6 million per mile that Homeland Security spent on fencing for recent upgrades in Arizona. The department said the increase is a result of inflation.

The Border Patrol says new fencing is needed to plug holes in defenses.

But congressional Democrats have vowed to oppose any money for more walls.

In a letter led by Sen. Kamala D. Harris, California Democrat, a group of the Senate’s most liberal members said costs could run to nearly $70 billion for construction, not including maintenance costs.

“Building a wall along the southwest border would divert critical resources away from more effective measures to ensure border safety, such as investing in port-of-entry security and procuring new technologies that monitor movements of people who try to cross the border,” the senators said in a letter addressed to the spending committee that will have to vote on funding.

The senators also said they want cuts in detention beds and deportation officers even below the levels of the Obama administration.

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