WASHINGTON (Reuters) - Republican Congressman Will Hurd - whose district spans 800 miles (1,290 km) of the Texas-Mexico border - on Friday criticized plans under consideration by the Trump administration to build walls and fences costing an estimated $21.6 billion to deter illegal immigration.  Reuters on Thursday revealed details of an internal report by the Department of Homeland Security (DHS) that estimated the cost of covering the entire border. It called for the first phase of construction to begin in San Diego, California; El Paso, Texas and the Rio Grande Valley. “Building a wall is the most expensive and least effective way to secure the border,” Hurd, whose district includes El Paso, said in an email. He said his district includes rough terrain where “it is impossible to build a physical wall.” The estimated price tag in the report is much higher than a $12 billion figure cited by Republican President Donald Trump in his campaign and estimates as high as $15 billion from Republican House Speaker Paul Ryan and Republican Senate Majority Leader Mitch McConnell. The border wall was one of Trump’s main campaign promises. Trump, who took office on Jan. 20, has vowed to make Mexico pay for it, but the United States’ southern neighbor has repeatedly said it will not fund its construction. Many congressional Democrats reacted strongly to the news of plans for the wall and its estimated price.  Senator Patrick Leahy of Vermont, the senior Democrat on the Senate Appropriations Committee, said in a telephone interview that he welcomed the debate in his committee over funding the wall.  “Instead of funding this costly and ineffective proxy for real action on immigration reform, we should be directing our resources toward finding cures for cancer, building schools for our children, feeding the hungry and rebuilding our bridges and our roads,” Leahy said. Five Democratic senators on Friday wrote a letter to Homeland Security Secretary John Kelly claiming that the money would be misspent.  The letter was signed by Senators Kamala Harris of California, Martin Heinrich of New Mexico, Tom Udall of New Mexico, Brian Schatz of Hawaii and Elizabeth Warren of Massachusetts. Warren, a star of the political left, was silenced in the Republican-controlled Senate on Tuesday evening for speaking out against Trump’s  attorney general nominee, Republican Senator Jeff Sessions. Sessions was confirmed on Wednesday.  The senators wrote, “We are extraordinarily concerned that President Trump’s executive order appears to require that you divert DHS funds meant for critical security priorities to instead fund the border wall.” They asked that Kelly respond to a series of questions, including how much funding will be diverted to cover costs for building the wall. Hurd said he had seen estimates as high as $40 billion for the barrier’s construction, citing a Massachusetts Institute of Technology study released in October. 