WASHINGTON — Jeff Sessions and Stephen Miller spent years on the political fringe in the nation’s capital as high-decibel immigration hard-liners, always warning about the dangers of open borders but rarely in a position to affect law or policy.

Now, Mr. Sessions, the attorney general and former senator from Alabama, and Mr. Miller, the president’s top policy adviser and former Senate aide to Mr. Sessions, have moved from the edges of the immigration debate to its red-hot center. Powerful like never before, the two are the driving force behind President Trump’s policy that has led thousands of children to be separated from their parents at the nation’s southern border.

It was Mr. Sessions who ordered prosecutors to take a new “zero tolerance” attitude toward families crossing into the United States, part of his plans to reshape the country’s law enforcement priorities to limit immigration. It is Mr. Miller who has championed the idea inside the White House, selling Mr. Trump on the benefits of a policy that his adversaries have called “evil,” “inhumane” and equivalent to child abuse or the internment of Japanese-Americans during World War II.