Editor's note: If you'd like an email notice whenever we publish Ross Ramsey's column, click here.

Julián Castro is already in the race for president. Beto O’Rourke is still deciding what office to seek, if any, in 2020.

But either still has time to take the road Lloyd Bentsen took in 1988, running for one of the state’s two U.S. Senate seats while simultaneously running on the national ticket.

It’s called the LBJ law — put in place in 1959 so Lyndon Baines Johnson, then the U.S. Senate majority leader, could run for re-election to the Senate at the same time he was on the Boston-to-Austin ticket behind John F. Kennedy.

It worked out for Johnson, if not for the Democrats. And it worked out for Bentsen, too, if not for Michael Dukakis, his presidential running mate.

Johnson won the vice presidency, a victory that put him in the White House when Kennedy was assassinated in November 1963. Bentsen won the Senate seat while he and Dukakis were losing the 1988 election to then-Vice President George H.W. Bush.

Maybe this is a portent, maybe a coincidence: Both senators were Democrats, and both were succeeded by Republicans. Republican John Tower lost the 1960 regular election to Johnson by a wide margin but emerged from a pack of 70 candidates in the 1961 special election that followed and won the runoff — a noteworthy upset in the Democrat-dominated political climate of the time.

Bentsen resigned his Senate seat in 1993, when President Bill Clinton appointed him to be Treasury secretary. The special election to replace him also drew a crowd — 24 candidates — and resulted in a runoff election in which a Republican beat a Democrat who’d been appointed by a Democratic governor.

In that first election, Tower beat William Blakely, who’d been appointed by Gov. Price Daniel Sr.; in the second, Republican Kay Bailey Hutchison beat Bob Krueger, who’d been appointed by Gov. Ann Richards.

The law that allows Texans to run for two national offices at the same time is known as the LBJ law because it was passed to allow Johnson to run for both the vice presidency and for re-election in 1960. In his case, and later in Bentsen’s, it meant an incumbent didn’t have to risk the seat he already had for a higher one he wanted. Johnson, as it turned out, didn’t need the protection; Bentsen did.

In 2020, neither of the two Texans potentially seeking the Democratic presidential nomination would lose an elective office if they lost the election. Both Castro and O’Rourke are in private life now and would return to private life after a defeat.

But they’re running in a field of more than a dozen candidates, a whirlpool of ambition that can drown even a strong political swimmer; just look at what happened to all of the initial favorites in the 2016 Republican presidential primary.

There is no guarantee that a Democrat can win a U.S. Senate election in today’s Texas. Nobody has done it since Bentsen’s win in 1988, three decades ago. But O’Rourke came close enough last November to raise Democratic hopes. He and others have talked about him running against John Cornyn, who is up for re-election in 2020. The El Pasoan hasn’t publicly said what he will do, though he has indicated that could come any day now.

The conversation so far has been presented as an either-or — that if O’Rourke runs for Senate, he’d be out of the presidential race. But both O’Rourke and Castro would be eligible to be on the Texas ticket for Senate and for president or vice president, if they so desired. Texas law prevents candidates from filing for more than one office, except for that LBJ exemption. And the exemption in the Texas Election Code is broader than history might indicate: “This section does not apply to candidacy for the office of president or vice-president of the United States and another office.”

Texans can run for president. Or for Senate. Or for both at the same time. Or, as history shows, for Senate and vice president at the same time.

That’s worth a political conspiracy theory or two, isn’t it?

In Castro’s case, it would take a change of heart to add a U.S. Senate run to his list of things to do. He’s already declared his candidacy for president. Like O’Rourke, he’s got an attractive resume for a veep candidate if he falls short; either or both men will probably be on the eventual nominee’s list of running mates if neither is the nominee.

Cornyn, without knowing who’ll be knocking on the door to challenge him, is already raising his profile, and also the balance in his political treasury.

He’s prepping for a hard race, whoever runs — even if it’s someone who’s also interested in other roads to Washington, D.C.