WASHINGTON — Homeland Security Secretary John Kelly told the Senate on Wednesday morning that he doesn't expect construction of a wall the full length of the U.S.-Mexico border — marking the abandonment of a central campaign promise by President Donald Trump.

"It's unlikely that we will build wall or a physical barrier from sea to shining sea," Kelly testified to the Senate Homeland Security Committee. "We're not going to build a wall where it doesn't make sense. But we'll do something across the Southwest border."

That's a direct contradiction of Trump's vow of a "big, beautiful wall" the full 2,200-mile length of the Southwest border. Trump has not personally indicated a willingness to abandon that promise, a cornerstone of his appeal to voters.

There are about 650 miles of fencing along the border, mostly in urban areas of California and Texas. Kelly has been seeking input from Customs and Border Protection officers. A typical response, he told senators, is: "Boss, if you could give me 27 more miles here, 16 miles here, I don't really care about the other 140 miles I'm responsible for."

Such "strategic fencing," he said, would deflect drug and immigrant smugglers away from cities, making them easier to catch.

"The barriers work. Technology also works," he said.

Democrats on the panel welcomed the about-face but sought reassurance that Trump himself is on board, and willing to abandon a promise not only to build a full-length wall and — implausibly — to make Mexico pay for it. Security experts call a full-length barrier wasteful and unnecessary, given the terrain along the 2,200-mile border. Texas officials in both parties reject the idea. Mexican leaders call Trump's demands offensive.

"The president has told me, Kelly, go do it. We need to protect the Southwest border in any way that makes sense. I have a lot of elbow room," Kelly told the committee. "The president knows that I am looking at every variation on the theme."

"I have a lot of elbow room," Kelly told the committee. "The president knows that I am looking at every variation on the theme."

That he said will include a combination of wall, high-tech fencing, surveillance and other techniques.

"I have no doubt that he will tell me to go do it," he said, even if his recommendation is for barrier well less than the full length of the border.

  • In this April 1, 2017, photo, a man in Nogales, Ariz., talks to his daughter and her mother who are standing on the other side of the border fence in Nogales, Mexico. Homeland Security Secretary John Kelly says arrests of people entering the United States illegally across the Mexican border plummeted in March. That's a signal that fewer people are trying to sneak into the U.S. (AP Photo/Rodrigo Abd)

  • Homeland Security Secretary John Kelly, Vice President Mike Pence, and others, listens during a listening session with President Donald Trump on opioid and drug abuse on March 29, in the Cabinet Room of the White House. (AP/Evan Vucci)

Sen. Claire McCaskill of Missouri, the senior Democrat on the committee, lauded Kelly for a reasonable approach but expressed skepticism that Trump will go along.

The sooner we stop this `we're going to build a wall from sea to shining sea,' the better," she said. "It's embarrassing. It's not going to happen. Everybody in Congress knows it's not going to happen…. It appears the only person who won't say it out loud is the president."

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Congress, she said, will never authorize funding for a 2,200-mile wall.

"You get it, we get it, but the president is so stubborn…. And by the way Mexico is not going to pay for it," McCaskill said. "The sooner the president gets some credibility on this the better."

The Homeland Security Department's specifications for the wall call for a barrier 18 to 30 feet, impervious to tunneling to six feet below the surface, and to assaults by torch or pick ax for a least a half-hour. Kelly said he doesn't know yet what it will look like, or how it would be painted, but indicated it will be tailored to local terrain and conditions, rather than uniform.

"I don't know that it will be made of. I don't know how high it will be. I don't know if it's going to have solar panels on its side and how it's going to be painted," he testified. "I will say this, that it's unlikely that we will build a wall or physical barrier from sea to shining sea…. I'm committed to putting it where the men and women [ who protect the border] say we should put it."

Arizona Sen. John McCain, a Republican, suggested that the term "wall" can cover electronic surveillance and as long as it's interpreted that way, the public will support it. He also noted that Trump's rhetoric has antagonized the Mexican public, creating a risk of elections that lead to an anti-American government south of the border.

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Kelly, a retired Marine general, cited a number of reasons for gaps in the wall.

He noted areas with "critical habitat," particularly in Big Bend. That's a remote national park in West Texas where the Santa Elena canyon soars 1,500 feet above the Rio Grande — a natural barrier no man-made fence could match.

Indian land in Arizona covers 75 miles of the border, Kelly said, and "that would be a place that would be unwilling, unlikely" to allow wall construction. Also, he said, "there are some eminent domain issues."

The Trump budget blueprint calls for hiring a squadron of 20 Justice Department lawyers dedicated to land acquisition, including through the process of forcing owners to sell their land for public purpose.

"We'll try to do as much as we can without those kinds of issues coming to a head," Kelly said.

Sen. Rob Portman, R-Ohio, also praised Kelly for affirming that "we're not going to build a wall where it doesn't make sense." 

He noted that he's been to Big Bend several times, including late last year – canoeing, Kelly interjected. "You're not going to build a wall on those canyon walls," Portman said, "and so we need reassure people that this is about an effective way to secure the border."