By Robin Emmott

LAREDO, Texas, Oct 12 (Reuters) - Texan mayors on the border with Mexico are threatening to take the U.S. government to court and are encouraging ranchers to do the same to block construction of a fence to keep out illegal immigrants.

Six mayors fear the planned fence, part of Washington's crackdown on illegal immigration, will hurt trade, split closely knit Mexican-Texan communities on both sides of the border and endanger wildlife.

Part of a federal plan to build 700 miles (1,120 km) of barrier along the entire border, the fence will also cut off Texan ranchers' access to the Rio Grande, the main source of fresh water in the region, the mayors say.

"We have to protect our property and we will do whatever is necessary to ensure there is no wall," said Laredo Mayor Raul Salinas, a former FBI agent.

A federal judge temporarily halted construction of part of the fence this week in Arizona on environmental grounds.

Since the failure of President George W. Bush's immigration reform plans in June, Washington has been focused on boosting border security and deporting illegal immigrants. Construction of the fence has already begun in California.

Eagle Pass Mayor Chad Foster said he had received hate e-mails from Americans outside Texas who accuse him of being soft on security. But the mayors argue there are better ways to stop illegal immigrants and drug traffickers.

"The perception in some parts of the United States is that you build a fence and then migration stops. The reality is that it will slow down migrants by three to four minutes," he said.

When asked about the fence, residents in heavily Hispanic southern Texas said few people support it.

Ranchers were also prepared to team up with the mayors in a potential suit over the fence, expected to be 153 miles (245 km) long in Texas, Foster said. Ranchers fear it will run through their land and block off the Rio Grande.

"Besides, we'll be ceding land to Mexico because we'll have to build the wall back from the Rio Grande, which is the border in Texas," said Del Rio Mayor Efrain Valdez.

A SEPARATE REPUBLIC

Cultural and economic ties to Mexico run deep in southern Texas and Mexico is a key trading partner for the state.

Until the mid-19th century, Laredo was in Mexican territory. In 1840, citizens of Laredo briefly formed their own republic -- the Republic of the Rio Grande -- with the northern Mexican states of Coahuila, Tamaulipas and Nuevo Leon.

Mayors advocate deepening and widening the Rio Grande to curb illegal immigration and drug trafficking, as well as increasing the number of Border Patrol agents and cutting back the Carrizo cane reeds growing on the river banks that allow people to hide in the river banks.

They complain those ideas are being ignored. "Washington is imposing this without consulting us, when we are the border communities," said Monica Weisberg-Stewart, a leader of the Texas Border Coalition that represents the mayors, judges, business leaders and citizens against the fence.

No one at the U.S. Customs and Border Patrol in Washington was immediately available to comment.

Any legal fight would take place in Texan courts, with mayors and landowners arguing that the federal government is trespassing on municipal and private land.

Legal experts say Texans are within their rights to take the government to court but that Washington can use its powers of eminent domain to seize land for the wall.

"Legally, mayors and landowners may not be able to block the construction but they can certainly delay it a great deal and make it very difficult for the federal government," said David Crump, a law professor at Houston University.

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