President Trump announced on Monday that he would address the nation from the Oval Office on Tuesday evening to discuss what he called the crisis at the southern border, and the White House said that later in the week he would travel to the border as part of his effort to persuade Americans of the need for a wall — the sticking point in negotiations with Democrats which caused a government shutdown. In an effort to justify his demands for a border wall, Mr. Trump has tried to paint the situation at the southern border as an imminent humanitarian and national security crisis, which Democrats and several immigration advocates argue is inaccurate.