Politicians all sing in unison when the question involves “litmus tests.”

They “never” apply such tests, politicians say. They don’t “believe in litmus tests.”

They all are lying.

I mention litmus tests because both major-party presidential nominees-to-be are about to select their vice-presidential running mates. Should Republican Donald J. Trump and Democrat Hillary Rodham Clinton require their VP picks to pass these litmus tests?

Sure they should.

In reality, though, there really is just one question that presidential nominees should always ask their VP choices: Are you ready to become president in the event something happens to me?

Trump is now apparently ready to choose between former House Speaker Newt Gingrich and Indiana Gov. Mike Pence. Both of those fellows no doubt would answer “yes” to the Big Question. The task for the campaign, though, is to persuade a majority of voters that they would be able to step into the job on a moment’s notice.

Clinton is facing a similar decision. Her field of hopefuls is much deeper than Trump’s. Sen. Tim Kaine of Virginia? Sen. Elizabeth Warren of Massachusetts? Hey, how about the guy who’s got the job now, Joe Biden? All of them surely would answer “yes” to the litmus test question.

Another possible Clinton choice has been rumored to be Julian Castro, the current housing secretary and former mayor of San Antonio. He’d answer “yes,” too, but some of us wonder whether he truly would be able to step into the box.

But when presidents are looking for people to fill key positions, you can damn sure bet that they have a set of policies and principles they demand of those they are considering.

Does that constitute a litmus test? Of course it does.

Consider the test that Ronald Reagan put his VP hopefuls through in 1980. Were they pro-life or pro-choice on abortion? That appeared to be a major question the hopefuls needed to answer correctly. Reagan settled on George H.W. Bush who, during his time in Congress, had been nicknamed “Rubbers” because of his strong voting record in support of organizations such as Planned Parenthood. Bush became an ardent pro-life candidate the instant he said “yes” to the Gipper.

Do you think Ronald Reagan had a “litmus test” that Bush had to pass? Absolutely!

So it will be this time around, just as it always has been.

If politicians say they don’t have “litmus tests,” they’re lying.