It seems almost impossible to imagine in today’s overheated, hyperpartisan environment. But there was a brief time in the weeks after the terrorist attacks of Sept. 11 , 2001, when congressional leaders of the two parties regarded one another not only as trustworthy allies but also as indispensable partners.

For then-Senate Minority Leader Trent Lott (R-Miss.), that altered reality crystallized one October afternoon in the Mike Mansfield Room of the Capitol as he was battling conservatives in his own caucus over the Bush administration’s request for expansive powers it said were needed to prevent another attack.

He sidled out a back door, tucked himself into a hallway alcove and dialed the one person he knew would help: Sen. Thomas A. Daschle (S.D.), his Democratic counterpart, who as it happened was having a similar argument with liberals in his caucus, just around the corner, in the Lyndon B. Johnson Room.

“This has to be done,” Lott recalled telling Daschle, who only five months before had unseated him as majority leader. “Meet me on the floor, and let’s do it right now.”

“I’ll see you there,” Daschle replied.

Over the next hours, the two huddled in the Senate chamber, strategizing and polishing what would be one of the most sensitive and controversial measures of the past decade. Ultimately, the legislation now known as the Patriot Act sailed through the Senate on a 98 to 1 vote.

That quiet meeting on the Senate floor was an example of what Republican President George W. Bush, quoting Democratic President Franklin D. Roosevelt, had called “the warm courage of national unity” in the wake of a national catastrophe.

“This is the unity of every faith and every background,” Bush had said at a Washington National Cathedral prayer service Sept. 14, 2001. “It has joined together political parties in both houses of Congress.”

What wasn’t clear then was that this brief, transcendent commonality of purpose would not extend beyond the exigencies of national security and a series of emergency measures to save endangered businesses such as the airline industry. Other differences not only didn’t heal, they festered.

That period was marked by “an unusual amount of effort to find consensus and bipartisan agreement,” said Richard A. Gephardt (Mo.), who was then the leader of the House Democratic minority. “But it was somewhat obvious that’s what we needed to do after such a traumatic, devastating event for the whole country, and a worry that other events would follow.”

By the month after 9/11, however, the two parties had begun to argue over — and this does have a familiar echo in 2011 — how to stimulate the fragile economy.

By the fifth anniversary, so much rancor had developed over the war in Iraq that even national security couldn’t bring the country together. A deep political schism was evolving.

And now, a mere decade after the event that was supposed to change everything, the two parties seem irreversibly at odds, not only over long-standing philosophical questions, such as the role and size of government, but also the more fundamental issues of our national values.

Nearly nine out of 10 Americans surveyed in the latest Washington Post/ABC News poll said the 9/11 attacks changed the country in a lasting way.

But how Americans view that change has shifted sharply in the past decade. Two months after the attack, 63 percent of Americans polled said the change had been for the better; now only 39 percent say that.

Persistent divide

So what happened? You could argue that the country’s return to bickering as usual, only more so, signifies some deep national character flaw. Or that it is proof of American resilience.

Either way, America’s political differences have long had a way of reasserting themselves, even under the most tragic and triumphant of circumstances.

“National unity doesn’t translate necessarily into political unity,” historian Richard Norton Smith said.

Smith said that less than a year after Pearl Harbor, FDR’s party came within a whisker of losing the House and gave up nine Senate seats. In 1918, a week before Armistice Day, Woodrow Wilson’s party lost the Senate to the Republicans — and with it, any chance of enacting his foreign-policy centerpiece, entry into the League of Nations.

What was true in 2001, and remains so now, is that the country is closely divided politically. But as Americans’ regard for their elected leaders has plummeted — particularly from the stratospheric levels reached after the 9/11 attacks — voters have grown less patient with them and more willing to kick them out.

Meanwhile, the gulf between the two parties in Washington has widened, as was demonstrated so vividly in August, when differences over fiscal policy took the nation to the brink of default on its debt.

Less ideological overlap

A study by political scientists Nolan McCarty, Keith Poole and Howard Rosenthal concluded that there is less ideological overlap between the two parties in Congress than at any other time since the late 1890s.

That means Washington has all but lost its ability to solve problems, much to the dismay of the electorate.

The result is that elections have produced a volatility unlike anything seen before in modern times. The White House has changed hands once, and control of both the House and Senate has swung twice since 9/11. That has reinforced the impulse for short-term political gain at the expense of anything that would lead toward cooperation and consensus.

Especially for the party in the minority, there is more incentive to gum up the works. As Lott, who is now a lobbyist, put it, every senator whose party has fewer than 50 votes wakes up each morning thinking, “If things go right today, I’ll be a committee chairman next year.”

And today’s achievement will be eclipsed by tomorrow’s setback. Even the May 2 killing of arch terrorist Osama bin Laden by Navy SEALs produced only a brief bump in approval for the commander in chief, President Obama, whose poll numbers stand at the lowest levels of his presidency.

“I don’t think any reasonable person would have expected that [post-9/11] unity to continue. Life moves on,” said former representative Lee H. Hamilton (D-Ind.), who co-chaired the 9/11 investigation commission.

What worries Hamilton more is something deeper in the system, something that he believes was made worse by the way national security trumped everything else in the years after 9/11.

“The system in many ways is not up to the challenge. We focus on a problem or two at a time to the exclusion of other problems,” he said. “We seem to have lost the capacity to deal with multiple challenges.”

Now of course, the economy is foremost.

As recently as December, when an NBC News/Wall Street Journal poll asked people to name the event of the past decade that had had the greatest effect on them, one-third said the 9/11 attacks. In the latest NBC/Wall Street Journal survey, only one in five said 9/11, and the percentage of those citing the economic recession rose to 46 percent, from 39 percent in December.

Becoming history

So the day that was supposed to change everything is getting smaller and smaller in the nation’s rearview mirror.

“Ten years in America is like dog years. This is now a historic event,” said Bill McInturff, a Republican pollster who helped conduct the two surveys. “However sad, this event is now a very long time ago.”

Polling director Jon Cohen contributed to this report.