There's no shortage of people telling Donald Trump he can't build a wall on the U.S.-Mexico border. And maybe, in the end, he won't do it. But at the moment Trump takes office, he will have the legal authority and the money he needs to get started on the wall. Yes, there will be obstacles — what's the over/under on the number of lawsuits that will be filed trying to stop it? — but the fact is, the law is already in place that will allow Trump to go forward.

As in other areas of immigration enforcement, Trump will be able to effect radical change simply by following the law. In this case, it is the Secure Fence Act, passed in 2006 with bipartisan support — 283 votes in the House and 80 in the Senate, including then-Sens. Barack Obama and Hillary Clinton.

The law ordered the Secretary of Homeland Security, within 18 months of passage, to "take all actions the secretary determines necessary and appropriate to achieve and maintain operational control over the entire international land and maritime borders of the United States."

Specifically, the law called for the usual mix of high-tech sensors, cameras, border checkpoints, vehicle barriers and other measures along the U.S.-Mexico border. And then it ordered Homeland Security to "provide for [at]* least 2 layers of reinforced fencing" and "the installation of additional physical barriers" for hundreds of miles on the southern border. For example, it ordered double fencing for the area "10 miles west of the Calexico, California port of entry to 5 miles east of the Douglas, Arizona, port of entry" — a span that would cover nearly the entire Arizona-Mexico border. The law ordered heavy fencing in other border areas as well.

The law's reference to "at least" a double-layer fence, plus its mention of "additional physical barriers," suggests that Congress specified the minimum amount that Homeland Security should do — not the maximum.

"The Secure Fence Act and the later amendment give full leeway to the president to determine what kind of fencing is appropriate, as long as it enhances security," noted Jessica Vaughan of the Center for Immigration Studies, which favors lower levels of immigration. "Clearly, action to upgrade the existing fencing and add additional barriers is authorized. If the president and his border security team want a wall instead of a fence, or an electric fence instead of a double chain link, or a fence of flowers instead of steel, they can do it."

The "later amendment" to which Vaughan referred was Congress' decision the next year, 2007, to partially back away from what it had done. Lawmakers passed an amendment sponsored by Texas Republican Sen. Kay Bailey Hutchison which said "nothing in this paragraph shall require the Secretary of Homeland Security to install fencing, physical barriers, roads, lighting, cameras and sensors in a particular location along an international border of the United States, if the secretary determines that the use or placement of such resources is not the most appropriate means to achieve and maintain operational control over the international border at such location." Hutchison's amendment gave Homeland Security the discretion to do less, but, as with the original law, did nothing to stop the department from doing more.

And that's where the Trump administration comes in. If Trump directs the secretary of homeland security to build a wall, then work can begin.

There's a lot of work to do. Last year, in response to questions from Republican Sen. Ted Cruz, DHS revealed that there are only 36.3 miles of double-layer fencing along the 1,954-mile Mexican border. Secretary Jeh Johnson told the Senate that 1,300 miles of the border, or 66.5 percent, have no fencing at all; 299.8 miles, or 15.3 percent, have vehicle fence; 316.6 miles, or 16.2 percent, have pedestrian fence, and 36.3 miles, or 2 percent, have double-layer fencing. Johnson also said there is no work being done on any additional fencing.

Would a Trump wall cover the entire border? Trump said many times during the campaign and since that, that would not be necessary. "We're going to build a wall," Trump told Sean Hannity Dec. 1. "Every once in a while, they'll say, Well, maybe it's not going to be totally full. Now, there are certain places you don't need a wall because you have — you know, you have mountains, you have other things, you have large and rather vicious rivers. But no, we're building a wall."

"Vicious rivers" aside, many immigration experts who favor tougher border enforcement agree with Trump that it would not be necessary to build a wall over the entire border. "I think another approximately 100 miles would make a big difference, according to what my Border Patrol sources describe," said Vaughan:

Then, I would recommend that the results be examined to determine if it makes sense to do more. Simultaneously to expanding and upgrading the existing barriers, I would recommend that the administration focus on changing the policies on how the government deals with the illegal crossers they are catching. The president should tell the Border Patrol to process and repatriate as many of these cases as possible, or re-direct the asylum seekers to the safe countries that they already traveled through. He should re-program resources to provide the Border Patrol with additional resources for temporary holding facilities until the surge of new arrivals slows down. He should allow the Border Patrol to resume patrolling the border and working transportation hubs and other proven types of enforcement. Most importantly, he should boost interior enforcement so that illegal aliens are not home free as soon as they clear the border. I think all of these steps would be more bang for the immigration control buck than additional barriers at a certain point.

Although Trump, once he takes office, will already have the authority to build the wall, he will face two daunting obstacles: money and litigation.

For funding, Trump could ask Congress to fund wall construction through the regular appropriations process. Or he could ask Congress to re-program billions of dollars that the Obama administration intended for other purposes. Or he could look for money elsewhere in DHS.

"There are funds provided annually to the Department of Homeland Security for construction in the Procurement, Construction, and Improvements Account that could be drawn on to begin construction along the border (perhaps with the need for some re-programming)," noted a senior Republican aide on Capitol Hill.

In addition, Trump could actually try to make Mexico pay for the wall, or at least for part of it. Trump's plan to do so — a fee on remittances and a fee on visas, which together Trump claimed might raise at least $5 billion — is actually one of the more detailed proposals released during the campaign.

Trump's final problem, and perhaps the most serious, will be litigation. The short version is, the Trump administration wall project will face lawsuits coming from every direction. From environmentalists. From property owners. From activist groups. From political opponents. From gadflies. From all around.

The lawsuits could be crippling. On the other hand, dealing with litigation intended to stop building projects is one of the few parts of the presidency that Trump has actual hands-on experience with in his earlier career. And building a wall is Trump's single most-repeated campaign promise. He has pledged to do it more times than anyone can count, most recently on Tuesday night, at his "thank you" appearance in Fayetteville, N.C., in which he vowed to "construct a great border wall." It's something many Trump supporters and aides believe he simply has to do. And he has the authority to get going, starting on day one.

* As it happens, it appears there is a typo in the law. The statute orders Homeland Security to "provide for least 2 layers of reinforced fencing," when it appears Congress meant to "provide for at least 2 layers…" The intent seems clear, but perhaps there could be some arguments about it in the future.