President Trump is so eager to complete hundreds of miles of border fence ahead of the 2020 presidential election that he has directed aides to fast-track billions of dollars’ worth of construction contracts, aggressively seize private land and disregard environmental rules, according to current and former officials involved with the project. He also has told worried subordinates that he will pardon them of any potential wrongdoing should they have to break laws to get the barriers built quickly, those officials said.