Presidential debates, even those with 10 candidates held some 15 months before Election Day, do matter.

Just recall the Aug. 11, 2011, Republican debate in Ames, Iowa, when Byron York of The Washington Examiner, in discussing the proposed combinations of spending cuts and tax increases to reduce the federal budget deficit, asked former Sen. Rick Santorum, R-Pa.: “Is there any ratio of (spending) cuts to (increasing) taxes that you would accept — 3-to-1, 4-to-1 or even 10-to-1?”​

Santorum replied, “No.”

Bret Baier of Fox News, followed up: “I’m going to ask a question to everyone here on the stage. Say you had a deal, a real spending cuts deal, 10-to-1, as Byron said, spending cuts to tax increases … Can you raise your hand if you feel so strongly about not raising taxes you’d walk away on the 10-to-1 deal?”

All eight candidates on that Iowa stage — former Massachusetts Gov. Mitt Romney, former House Speaker Newt Gingrich of Georgia, former Minnesota Gov. Tim Pawlenty, former Utah Gov. Jon Huntsman, then-Rep. Ron Paul of Texas, then-Rep. Michele Bachmann of Minnesota, former Godfather’s Pizza CEO Herman Cain and Santorum — raised a hand.

Forget all the speeches about the national debt and federal deficits being a mortal threat to America’s national security. Forget that President Ronald Reagan, of sainted memory — agreeing to $3 in future spending cuts for $1 in tax increases — signed the largest tax increase in U.S. history.

The Republican Party, voters learned in that August debate, was openly unserious about deficit reduction and theologically uncompromising on the absolute priority of tax cuts.

Because in life we rarely get a second chance to make a first impression, the governor of the state that had, under his leadership, produced one-third of the new jobs in the entire United States during the first decade of this century — a man who entered the 2012 race leading in all national polls — is primarily remembered for becoming a late-night TV punch line.

In Rochester, Mich., on Nov. 8, 2011, then-Texas Gov. Rick Perry said, “I will tell you, it’s three agencies of government when I get there that are gone: Commerce, Education and the — what’s the third one there? Let’s see.”

He struggled to remember. Romney tried to help, jokingly suggesting the Environmental Protection Agency.

Finally, after a painful, wince-inducing 28 seconds, searching for the Energy Department, Perry surrendered: “I would do away with the Education (Department), the Commerce (Department) and let’s see. I can’t. The third one, I can’t. Sorry. Oops.”

In the 2016 opening round in Cleveland, the political question is: Who among the lower nine will bell the cat by most directly challenging the surprise front-runner, billionaire Donald Trump?

This is tricky territory, because in a multicandidate contest when candidate A delivers a hatchet job — even a believable one — on candidate B, the eventual beneficiary is almost always candidate C, D, E, F or G.

My bet is that the opening-night strategy of New Jersey Gov. Chris Christie, whose preferred role as the no-nonsense, tell-it-like-it-is guy has been pre-empted by Trump, will be to confront and to expose and provoke the front-runner.

But you can be sure that Christie, if I’m right, will not be the only one on the stage throwing down the gauntlet to The Donald.

Presidential debates should be seen for what they are: a public opportunity for citizens to conduct joint job interviews of would-be chief executives wherein voters can simultaneously compare and evaluate the qualities of intellect, philosophy, character and personality they seek in a national leader.

You can be certain that the 2016 race will be permanently changed by something learned in Thursday’s Cleveland debate.

Mark Shields is one of the most widely recognized political commentators in the United States. The former Washington Post editorial columnist appears regularly on CNN, on public television and on radio. Click here to contact him, or click here to read previous columns. The opinions expressed are his own.