President Donald Trump arrives at Miramar Air Corps Air Station, Tuesday, March 13, 2018, in San Diego. Credit: Evan Vucci | AP

President Donald Trump traveled Tuesday to opposition territory — California — to fire up support for a wall along the U.S.-Mexico border, a project he’s having trouble persuading Congress to fund.

Trump inspected eight wall prototypes constructed in the desert south of San Diego as he fights to overcome resistance from Democrats and skepticism from some Republican lawmakers over the cost of a barrier that was a central promise of his presidential campaign. The White House has requested $18 billion for the project.

“If you don’t have a wall system, we’re not going to have a country,” Trump said as he pointed to a scrap-metal barricade that currently delineates a portion of the southern border. “The fence is not strong enough,” he said, adding he preferred walls that are at least in part “see-through.”

A guard standing in a tower on the Mexican side of the border fence watched through binoculars as the president toured the area.

The wall is one of the last major sticking points in negotiations over funding the federal government for the remainder of the year once a stopgap spending bill expires on March 23. Lawmakers haven’t decided what to do about it.

Trump’s tour of the sample wall sections at Otay Mesa Port of Entry stirred plans for protests in a state where the project is deeply unpopular, as is the president. A Public Policy Institute of California poll released in January found that 73 percent of adults in the state opposed the wall while just a quarter supported it. Trump lost the state by a two-to-one margin in 2016, the worst performance of any modern Republican presidential candidate.

Trump has maintained that he expects Mexico to ultimately pay for the wall, perhaps as part of a renegotiation of the North American Free Trade Agreement. Mexican President Enrique Pena Nieto has repeatedly rejected the idea. The issue has helped sour relations between the two leaders.

He also raised the possibility in a tweet Tuesday of justifying the cost of the wall based on savings in government spending which immigration opponents anticipate from the boundary. The Twitter message cited estimates by the Center for Immigration Studies, which favors lower levels of immigration.

“According to the Center for Immigration Studies, the $18 billion wall will pay for itself by curbing the importation of crime, drugs and illegal immigrants who tend to go on the federal dole,” Trump tweeted.

The National Academies of Sciences, Engineering and Medicine concluded in a 2016 report that immigration in recent decades has raised overall U.S. economic growth.

First-generation immigrants are more costly to governments, mainly at the state and local levels, than are native-born Americans, in large part because of the cost of educating their children, the report found. However, as adults, those second-generation immigrants are among the strongest economic contributors to the country and pay proportionately more in taxes than the rest of the native-born population.

The trip is Trump’s first presidential visit to California, the nation’s most populous state and an engine of growth that, if it were a country, would rank as the world’s sixth-largest economy. He will also headline a Republican National Committee fundraiser in Beverly Hills, where tickets cost as much as $250,000.

California Gov. Jerry Brown, D, sent Trump a back-handed invitation on Monday suggesting a change of itinerary to add a visit to a construction site for what he said is the nation’s first true high-speed rail line.

“In California we are focusing on bridges, not walls,” Brown wrote.

Trump struck a different tone about the state’s needs, saying “the state of California is begging us to build walls in certain areas.”

Brown’s government and environmental advocacy groups have sued to block construction of the wall, claiming that the administration acted illegally when it waived certain environmental rules. U.S. District Court Judge Gonzalo Curiel, whom Trump attacked during his 2016 campaign, said in an initial ruling last month that the administration had legal authority to proceed with the project.

The animosity between the Trump administration and California Democratic leaders was demonstrated on Monday, when the acting director of Immigration and Customs Enforcement brushed back House Minority Leader Nancy Pelosi and Senat. Dianne Feinstein for criticizing his agency’s work.

Pelosi issued a Feb. 28 statement after ICE conducted what she called “sweeping raids across Northern California.” She said the arrests of more than 150 people were “intended solely to terrorize innocent immigrant families.”

The acting ICE director, Thomas Homan, told reporters at a briefing on Monday that Pelosi’s remarks “were just beyond the pale.”

The White House requested $25 billion for the wall as part of a longer list of demands last month in exchange for granting legal status to undocumented immigrants brought to the country as children. Democrats rejected the deal over proposed restrictions on legal immigration. Trump has said that he believes the wall would cost about $18 billion and that the rest of the money can go toward other border security projects.

U.S. Customs and Border Protection solicited bids last year using existing border security funding to build eight wall prototypes and awarded contracts to six private companies for mock-ups that are each 30 feet long and as much as 30 feet high. The agency is testing how well each prototype withstands attempts at breaching, climbing, digging and other factors.

Caddell Construction Co. of Montgomery, Alabama; W.G. Yates & Sons Construction Co. of Philadelphia, Mississippi; Fisher Sand & Gravel Co. of Tempe, Arizona; and Texas Sterling Construction Co. of Houston each won contracts to build concrete prototypes, the agency has said. Additionally, Caddell, W.G. Yates, KWR Construction Inc. of Sierra Vista, Arizona, and ELTA North America Inc. of Annapolis Junction, Maryland, won bids for mock-ups made from materials other than concrete.

Berkeley, Oakland and other cities have sought to stop doing business with firms involved with the wall, prompting the Associated General Contractors of America to ask Attorney General Jeff Sessions to sue to prevent states and localities from denying contracts or divesting from companies that participate in the barrier’s construction. There’s been no action on the contractor association’s request, Brian Turmail, a spokesman for the organization, said on Monday.

With assistance from Toluse Olorunnipa