WASHINGTON -- Disclosures about National Security Agency cyber-spying on millions of Americans vividly illustrates how the federal government’s check-and-balance system is out of balance.

Despite periodic attempts to assert itself, the legislative branch over time has settled into a secondary role to the executive branch on questions of national security. The dominance of the executive branch in the nuclear age – when presidents claimed the need to act on a moment’s notice – continued into the age of terrorism with the claimed need for vast new spy powers handed over by Congress with the Patriot Act and renewed and extended ever since.

Lawmakers offered little resistance to American intervention in Libya two years ago, or to the use of American troops in central Africa and Uganda. Nor was there much demand for changes in the use of drones aimed at suspected terrorists in foreign countries, even after the administration disclosed last month that four Americans had been killed by strikes abroad.

Bowing to the president in the interest of protecting the nation has been commonplace for a century, ever since the U.S. became a major international player and had to react quickly to crises. “Presidents assumed power and got away with it,” said Stephen Hess, a presidential historian at Washington’s Brookings Institution who worked with four presidents.

Some of Congress’ inability to act like a co-equal branch of government is rooted in the institution’s nature. The president can act quickly and speak with one voice. Congress, divided between two parties and two chambers and featuring political factions ranging across the spectrum, cannot.

The executive branch has another built-in edge: The public routinely supports the president’s ability to act decisively in times of crisis.

Even those who once urged more congressional advice and consent see events differently from inside the Oval Office.

In 2006, for example, Denis McDonough urged a more active role for Congress, in a study for the Center for American Progress, a liberal research group.

“Recent news headlines that the National Security Agency is collecting the phone records of tens of millions of Americans without the knowledge of key congressional committees underscores the need for Congress to serve as the American public’s watchdog in overseeing intelligence agencies,” the 2006 study said. “Congress today has been negligent, with profound implications for the safety and security of America.”

Now White House chief of staff to President Barack Obama, McDonough argues that safeguards have been established so Congress has a stronger role.

Indeed, members of Congress have had access to information about the spy programs. But few have shown up to read the material.

“It’s the perfect example of Congress handing over power to the executive branch and failing to keep up with it,” said Jim Harper, a former congressional counsel and now director of information policy studies at the libertarian Cato Institute.

Congress thought it had given itself nearly equal billing by passing the 1973 War Powers Resolution. Approved after years of political conflict over Vietnam, it aimed to place new restrictions on presidents’ military initiatives.

But “every president has taken the position that it is an unconstitutional infringement by Congress on the president’s authority as commander in chief,” Richard Grimmett, a Congressional Research Service international specialist, said in a report. Courts have not directly addressed the issue.