IN the continuing fiscal negotiations between President Obama and House Republicans, both sides have, from the very beginning, agreed on one point: Taxes on the middle class must not rise. But maybe it’s time to reconsider this premise. An unwavering commitment to keep middle-class taxes low could be one reason the political process has become so deeply dysfunctional.

Let’s start with the problem: the budget deficit. Under current policy, the federal government is spending vastly more than it is collecting in tax revenue. And that will be true for the next several decades, thanks largely to the growth in entitlement spending that will occur automatically as the population ages and health care costs increase. As a result, the ratio of government debt to the nation’s gross domestic product is projected to rise, substantially and without an end in sight.

That can happen for a while, or even a long while, but not forever. At some point, investors at home and abroad will start questioning our ability to service our debts without creating steep inflation. It’s hard to say precisely when this shift in investor sentiment will occur, and even whether it will strike in this president’s term or the next, but when it does, it won’t be pretty. The United States will find itself at the brink of an unprecedented financial crisis.

Republicans and Democrats agree on the nature of the problem, but they embrace very different solutions. My fear is that both sides are engaged in an excess of wishful thinking, with a dash of mendacity.