I don’t know about you, but when I hear that Congress has a rock-bottom 15 percent approval rating, I wonder: How could the number possibly be so high?

There are many reasons to hold Congress in contempt.

There’s the shameless back-scratching, logrolling, moneygrubbing, and pork-barrel spending that is as old as the institution itself, but which has gotten a major boost in the post–Citizens United era.

Then there’s the polarization and institutional dysfunction, rooted in rule changes, gerrymandering, and the ideological self-sorting of the parties in recent decades.

But as frustrating as these pathologies can be, there’s another one that may in the long run prove to be even more deleterious to the health of the American political system. This is the unwillingness of Congress as an institution to push back against the executive branch’s ever-more expansive efforts to wage war without legislative oversight.

I’m not talking about surveillance, digital and otherwise, about which there has been a public outcry, leading to some tentative moves in Congress to place limits on executive power.

I mean war-making in the precise sense: sending military advisers into war zones, launching missiles, and deploying combat troops.

The Constitution understandably gives the president a fair amount of leeway in matters of war and peace. There will be situations in which a sneak attack or a quickly developing series of complicated events requires a military response so rapid that congressional consultation, debate, and a vote in favor of an official declaration of war may be impractical. In such cases, the president may be justified in initiating hostilities without congressional oversight, despite the Constitution’s clear placement of the power to make war in the hands of Congress.

But such situations are supposed to be the exception to the rule, not the rule itself.

And yet, they have become the rule. Yes, presidents have contributed to this development by making ever-greater claims to executive war powers. The constitutional framers anticipated that this would happen. That’s why they constructed an institutional structure of competing branches — so that the ambitions of each branch of government would check the ambitions of the others.

What they didn’t anticipate is that one of those branches — the legislature — would choose to passively accept executive aggrandizement on the most serious issues facing the nation.

Congressional shirking of responsibility arguably began during the Cold War. Neither the Korean War nor the Vietnam War were formally declared. That led to the 1973 passage of the War Powers Resolution, which was designed to force presidents to seek formal congressional approval of military actions lasting longer than 60 days. (Ronald Reagan’s small wars, as well as most of Bill Clinton’s, stayed on the short side of that line.)

Uncertainty about how long it would take to expel Saddam Hussein’s army from Kuwait, as well as the enormous scale of Operation Desert Storm, led George H.W. Bush to seek and receive congressional authorization for the first Gulf War in 1991. It was Congress’ most intensive debate about war and peace since 1942.

Things have been going downhill ever since.

Congress provided a blanket, open-ended authorization for a war against terrorism after the Sept. 11 attacks. It did the same for Iraq in October 2002 — five months before the bombing of Baghdad began. Remarkably, there was no serious congressional debate.

The contrast with Great Britain, America’s primary ally in taking on Saddam Hussein, is revealing. While British prime ministers are not constitutionally obligated to seek legislative approval for war, the House of Commons debated the imminent invasion of Iraq for nearly nine hours on March 18, 2003, while the nation followed the arguments and final vote on television.

On through seven years of war and occupation in Iraq — and nearly 13 years of the same in Afghanistan — Congress has largely deferred to Presidents Bush and Obama. The same holds for the latter’s undeclared (and disastrous) mini-war in Libya.

Which brings us to America’s recent military intervention in northern Iraq to defend the Kurds and others against ISIS — an intervention that just might lead to bombing and troop deployments over the border in Syria as well.

I’m not opposed in principle to using military force to decimate this group of cold-blooded maniacs. In the end, it should come down to a clear-eyed assessment of the threat they pose to the United States. Over the past few weeks, I’ve heard compelling evidence on both sides of that argument.

There’s just one problem: Bombing a nation — even when it’s mainly to attack substate actors operating within it — is unquestionably an act of war. And debates about whether to go to war should be taking place in Congress, with our elected representatives taking a stand one way or another. The refusal to take that stand is a monumental evasion of Congress’s constitutionally delineated responsibility.

That this shirking of responsibility is a product of abject cowardice and self-protectiveness makes it especially contemptible.

Serving in Congress has become so cushy that our representatives would rather protect their jobs than take a risk in defense of the public good or the prerogatives of their branch of government. Sure, they’ll support demagogic partisan stunts, like House Speaker John Boehner’s lawsuit against President Obama. But actually take responsibility for war and peace? Nah. Much better to stay silent now and then reserve the right to attack the president if military action goes badly — or benefit from the outpouring of national good will if it goes well. Either outcome is more likely to ensure re-election than making an uncertain judgment call now, before all the consequences of military action, or nonaction, are known.

This isn’t the way our institutions are supposed to function. But it is undeniably the way they do function. And all of us are the worse for it.