Grover Norquist, president of Americans for Tax Reform, in 2013. (Alex Wong/GETTY IMAGES)

NEW JERSEY Gov. Chris Christie hardly stands out in this crop of Republican presidential candidates for putting ideology above reality. But given his desire to be seen as a truth-teller of unusual courage, it’s noteworthy that he succumbed to pressure this week and signed anti-tax crusader Grover Norquist’s pledge to “oppose and veto any and all efforts to increase taxes.” In so doing, Mr. Christie, whose Web site is effusively spangled with the phrase “Telling it like it is,” joins neurosurgeon Ben Carson , Sen. Ted Cruz (Tex.), businesswoman Carly Fiorina, former Arkansas governor Mike Huckabee , Sen. Rand Paul (Ky.), former Texas governor Rick Perry , Sen. Marco Rubio (Fla.) Wisconsin Gov. Scott Walker, Louisiana Gov. Bobby Jindal, Ohio Gov. John Kasich and former senator Rick Santorum (Pa.) in kneeling before Mr. Norquist’s make-believe anti-tax theology.

Why do we say make-believe? Here are some facts for the truth-tellers. Federal spending averaged 20.1 percent of gross domestic product from 1965 to 2014. With the baby boomer generation retiring, the population aging and health costs rising, the Congressional Budget Office projects that government spending will grow to 25.3 percent of GDP by 2040.

At that time, federal revenue is projected to equal about 19.4 percent of GDP absent any policy changes. There is, in other words, a vast budget gap that will need to be filled. Unlike his opponents, Mr. Christie has proposed specific benefit cuts that would narrow the gap somewhat. But neither his proposals, nor any other, can close the gap entirely in the absence of increased revenue. Trying to do so would leave the government paying pensions and rising interest costs (as it borrowed more and more) and devoting little or nothing to the other things Americans expect from government: defense, roads, bridges, basic scientific research, national parks and more.

Political bravery would be admitting this reality — not just the attenuated version the GOP base wants to hear — and refusing to pre-reject an entire range of policy options to deal with it. Some candidates have managed this. Though no fan of raising taxes, former Florida governor Jeb Bush, for example, has never signed the pledge through several previous campaigns for office. That is the only responsible position. To sign a pledge is to make a reckless promise that locks politicians into an arbitrarily restrictive budget policy, no matter what circumstances time brings, and ignores the reality that is bearing down on the nation.