Their political affiliations may differ, but many of the ranchers along the Rio Grande are angry about the same thing — they don’t want to lose their land to the federal government.

“This isn’t about Republicans or Democrats,” said Rusty Monsees, who owns 21 acres steps from the river. “We’re Texans.”

Although he supports Donald Trump’s proposed border wall, he says he is worried about being forced to sell his land cheap in order to accommodate the new barrier.

He is one of thousands of landowners who are gearing up for a legal battle along the 138 meandering miles of the Rio Grande as US Customs and Border Patrol begins the “Border Infrastructure Project” to survey lands needed for a border wall.

The landowners who refuse to sign on to the project and sell their land risk being sued by the federal government and having their land seized through eminent domain.

“It’s going to be a long, drawn-out fight,” said Charles McFarland, a Houston lawyer who has been locked in a decade-long battle with the federal government on behalf of a client who owns 140,000 acres in the Rio Grande Valley and refused to sell part of their land when the Bush administration drew up plans to build a barrier.

During the Obama era, Monsees lost 3 acres of land under eminent domain — and was paid a paltry $500 an acre.

McFarland is expecting an influx of new clients as surveys get underway for the Trump structure.

“Once they have their surveys, they will file petitions to acquire title,” McFarland said.

Fred Cavazos, who owns 77 riverfront acres in Mission, told The Post he will go to court to protect his land.

The proposed wall will cut off his access to the river, where he has 30 tenants who rent vacation cabins on land that has been in his family for hundreds of years, said Cavazos, sitting in his wheelchair near a cottage where he learned to fish as a child.

“If they cut off access to my land, and I can’t rent out the cabins, I don’t know how I’ll survive,” said the 71-year-old paraplegic.

Cavazos is a descendant of Jose Narciso Cavazos, who received the largest land grant — 600,000 acres — from the King of Spain in 1792. Over the years the parcel has shrunk through land seizures and family disputes as the region changed hands from Spain to Mexico to Texas to the US.

His cousin, Rey Anzaldua, 74, a retired customs officer, said, “Our family has been here since before there was a United States. If they build a wall, we are going to tear it down.”