WASHINGTON — As a senator, Jeff Sessions was such a conservative outlier on criminal justice issues that he pushed other Republicans to the forefront of his campaign to block a sentencing overhaul, figuring they would be taken more seriously.

Now Mr. Sessions is attorney general and need not take a back seat to anyone when it comes to imposing his ultratough-on-crime views. The effect of his transition from being just one of 535 in Congress to being top dog at the Justice Department was underscored on Friday when he ordered federal prosecutors to make sure they threw the book at criminal defendants and pursued the toughest penalties possible.

“This is a key part of President Trump’s promise to keep America safe,” Mr. Sessions said on Friday as he received an award from the New York City police union to mark the beginning of National Police Week.

Given Mr. Sessions’s long record as a zealous prosecutor and his well-known views on the dangers of drug use, his push to undo Obama-era sentencing policies and ramp up the war on drugs was hardly a surprise. But it was still striking, because it ran so contrary to the growing bipartisan consensus coursing through Washington and many state capitals in recent years — a view that America was guilty of excessive incarceration and that large prison populations were too costly in tax dollars and the toll on families and communities.