Without worrying about his path to the White House, he is likelier to emerge as the Senate’s leading critic of reckless foreign policy, helping to build an Inside-the-Beltway counterweight to the bipartisan establishment of overconfident interventionists.

The rise of facial-recognition technology, license plate readers, self-driving cars, and other “smart” consumer products makes future privacy showdowns an inevitability. No one is better positioned to capitalize on public anxiety or outrage.

If Paul puts his head down and sets aside his ambitions beyond the Senate, he’ll find that another four or eight years burnishing a reputation as a pragmatic man of principle would actually mitigate many of his weaknesses as a national figure.

Lots of Ron Paul supporters forgave him his personal prickliness, his lack of charisma, his kooky affect, and even the abhorrent racism that sullied his newsletter business because he proved, through sincere consistency across many years, that he really was different than other politicians; that for all his downsides, he couldn’t be co-opted by the Washington establishment, or the military-industrial complex, or the Israel lobby, or the Wall Street banks and hedge funds.

Rand Paul hasn’t yet proved himself as solid. He hasn’t had the time. His core constituency wasn’t quite sure what compromises he would make to win the White House. And voters who felt heartened by some of his principled stands despite disagreeing with his world view weren’t going to back a man they hardly knew, especially a man who, like Mr. Darcy, seems at first to rub people the wrong way.

To rise rapidly in U.S. politics, one needs the charisma of a JFK or a Ronald Reagan or a Bill Clinton. Pricklier, more awkward sorts, like John McCain and Hillary Clinton, aren’t out of luck. They just need to stick around long enough for Americans to get comfortable with their personalities. (Familiarity is oddly helpful to politicians. Look at Trump. Millions forgive him behavior that would sink anyone else. “Oh, that’s just The Donald,” they say, “being himself.”)

As a civil libertarian who values Paul’s stands on drones, NSA surveillance, policing, and other issues besides, my biggest fear is that his quashed presidential aspirations will sour him on politics, that he’ll return to ophthalmology, and that Kentucky voters will replace him with another Mitch McConnell.

Doesn’t he seem like he would tire of flying to Washington, D.C.?

But if he sticks it out, is deft in marrying principle and pragmatism, gradually improves his political team, and works to advance his agenda without taking shortcuts, I can imagine this scenario: he retires as a historically consequential senator who fought a succession of executive-power-hungry presidents, helped to constrain U.S. foreign policy, limited the government’s ability to surveil its citizens, and helped transform its scandalous approach to criminal justice and mass incarceration.