US President Donald Trump has continued to promise to build a wall along the US-Mexico border. But he'll have to get through a lot of obstacles first.

The dust rises with the softest touch. It clings to your clothes, sucking the moisture from your skin, making breathing an effort.

When the wind blows - and it is blowing hard today - the earth billows up and races across the desert, rare clouds under a thirsty sky.

These are the border lands of west Texas. The only water to be seen is the Rio Grande, the river which separates Mexico and the United States.

There is plenty of support for President Donald Trump in this big, red Republican state but, along the frontier, precious few are fans of his wall.

Media playback is unsupported on your device Media caption Will Hurd on why he doesn't want a wall

"I don't think people understand that the border is a very big place. In my district alone I have 820 miles (1,320km)," Congressman Will Hurd tells the BBC as he surveys the landscape.

The Republican knows a thing or two about security. For nearly a decade he worked for the CIA in the Middle East, Afghanistan and elsewhere. Now he finds himself on the home front, resisting his own party's president.

"Building a wall from sea to shining sea is the most expensive and least effective way to do border security," says Mr Hurd.

Instead, he suggests, the US should invest in improving intelligence on drug-smuggling gangs as well as hiring extra border patrol agents and installing more cameras and motion detectors.

"These are all things that are a better use of American taxpayer dollars, and we're going to see quicker and more successful results," he insists.

Mr Hurd and others like him are a problem for the president, who needs Republican lawmakers to approve funding for the wall if it is to become a reality.

Of course Mr Trump continues to insist that Mexico will eventually reimburse the US for the cost of construction (estimates of the price tag vary astonishingly between $4bn (£3.1bn) and $66.9bn, although he has yet to give a coherent and specific explanation of precisely how this will happen.

Nonetheless, the federal government is pressing ahead with its plans for the wall.

More than 200 companies have reportedly responded to an invitation from the Department of Homeland Security (DHS) to submit designs.

Interested vendors were briefed that the government anticipated "procuring concrete wall structures, nominally 30ft tall, that will meet requirements for aesthetics, anti-climbing, and resistance to tampering or damage."

A "beauty contest" is planned near the Californian border city of San Diego sometime this summer, when a handful of companies will be asked to build prototypes for the government - possibly even the builder president himself - to inspect.

The resulting bids range from the sensible to the surreal to the satirical. Sometimes it is difficult to be certain which is which.

Image caption A portion of the current fence near El Paso, Texas

An Illinois firm, Crisis Resolution Security Services, has promised a wall "as pretty as the Parthenon" that makes a "powerful statement of the determination of the American people to defend their nation and its Anglo-Saxon heritage, Western culture and English language".

These are not words likely to win over the millions of Spanish speakers on both sides of the border.

A zigzag design by one Rod Hadrian also invites inevitable comparisons with a classical structure, if only for the name of Hadrian's Wall.

California's Concrete Contractors Interstate suggests an "aesthetically pleasing" wall of polished concrete, embossed on both sides with artwork showcasing "various cultures and communities" along its route.

Gleason Partners of Nevada wants a wall adorned with solar panels, while a Pennsylvania firm, Clayton Industries, has even drafted a design which includes chambers to store nuclear waste.

And then there is PennaGroup of Fort Worth, Texas. Its boss, Michael Evangelista-Ysasaga, is Latino. Eighty per cent of his staff are Mexican-Americans. Throwing his hat into the ring has seen him branded a traitor to his people.

He says he has received "at least a dozen" death threats. Security has been beefed up. On the morning we visit him at his office he plays us the latest offensive message on his voicemail, from an anonymous woman.

Image caption Michael Evangelista-Ysasaga says he decided to make a bid after hearing of dangerous options from other firms

"Yeah, if this is that Michael Ysasaga from the PennaGroup," she begins, slowly and deliberately, "I know that y'all are trying to build that wall and I think it's a disgusting disgrace and you are a traitor.

"You'd better watch your back. There are plenty of people out here like me that would love to see you get... hurt."

Mr Evangelista-Ysasaga says the woman and others like her have got him all wrong. He is only bidding for the wall, he insists, because he was disturbed by rumours that rival firms were proposing electrified fences or razor wire.

It was important, he says, to offer a humane, non-lethal option.

"The border wall is a reality and if it's going to be done we need it to be done right," he told the BBC.

The PennaGroup chief executive hopes that construction will be followed by comprehensive immigration reform to provide a path to citizenship for the estimated 11 million undocumented immigrants (although he reckons the true number may be as high as 18 million).

"The right has coalesced, they've rallied around the security issue. The left has rallied around immigration reform and everyone is just yelling at each other and there's no productive conversation going on. Thus the death threats," he says.

Image copyright PennaGroup Image caption Pennagroup is considering various options in their wall proposal

The project faces other obstacles too.

If a wall is to be built from the Pacific Ocean to the Gulf of Mexico through California, Arizona, New Mexico and Texas, it will require land, much of which is in private hands.

Some 650 miles of the roughly 2,000-mile border is already fenced. President Trump has suggested that natural obstacles might mean other sections can be left untouched, but even so battles loom.

In sultry Starr County, not far from the spot where the Rio Grande narrows at El Paso de la Mula (the Pass of the Mules), Noel Benavides is standing in the middle of a forest clearing.

Every so often the birdsong is drowned out by the roar of an American patrol boat on the river. Armed US border agents creep through the undergrowth in camouflage.

"This property has been in my wife's family since 1720," Mr Benavides tells us.

Or to put it another way, the family has owned this land when it was Spanish, Mexican, Texan, Confederate and part of the United States.

Not for much longer, frets Mr Benavides. He first received notice that the government was interested in taking part of the land to improve border security in 2008, under President Obama.

But nothing much happened until President Trump came to power when the demand was renewed in a letter of condemnation.

"They want to purchase 5.7 acres," he tells us, "It's a strip, 60 feet wide by a mile or more in length."

Image caption Noel Benavides looks over the Rio Grande

The strip through which the government intends to route the wall runs through the heart of Mr Benavides' land near the town of Roma and would leave roughly half of the property in "no-man's land" on the southern side, with an effective value, he says, of zero.

"We need access to the river. We've got water rights," says Mr Benavides, who doesn't think the wall will do anything to improve security.

"We go back in history, it's never worked. You've got a 20 foot wall? Somebody's going to build a 21 foot ladder to go over it."

So the wall faces a struggle for land, funding and political support.

It had looked like a battle over the bill would come to a head this week, perhaps even forcing a partial shutdown of the government.

The White House backed down postponing the fight until later in the year.

Critics say that simply delays the project's death but President Trump, in tweet after tweet, continues to insist that his wall will be built - and in the dusty desert the project is creeping slowly forward.