“This was indeed an intimidating time,” Mr. Feingold writes in his new book, “While America Sleeps” (Crown Publishers), his recounting of the historic Congressional period after the Sept. 11 shock through the rise of the Tea Party conservatism that led to the loss of his own Senate seat in November 2010. Best known as the chief Democratic architect of the campaign finance law, Mr. Feingold assembles a narrative of how the terrorist attacks, the deadly anthrax assaults and, later, the sniper killings of 2002 in the Washington region created a climate of fear that ultimately led not only to the antiterrorism law, but also to the war in Iraq and ugly political attacks. The pleasant Capitol Hill neighborhood that he inhabited became an armed camp reminiscent of another anxious period in American history when civil liberties were at risk. “The combination of constant security fears with the weightiness of the responsibilities made this what had to be one of the most tense and frightening times on Capitol Hill since the Civil War,” he writes. It was the searing combination of events that he blames for knocking Congress off kilter, from the days of political unity immediately after Sept. 11 to sharp divisions and distrust, an atmosphere that he says allowed the administration of President George W. Bush to successfully push its agenda despite severe doubts among lawmakers. “When those things started coming, we got into this odd position of thinking about the problems in a completely wrong way,” Mr. Feingold said in an interview. Since his defeat, Mr. Feingold, 58, has been teaching, organizing a new progressive group and writing his book, which tells the familiar story of the buildup to the war with Iraq despite the absence of a connection between that nation and the Sept. 11 attacks. But he goes behind the closed doors of the Democratic caucus as lawmakers debated using force against Iraq, and he even provides a peek into the secret briefings members had as the emotional case for war was being made in public. The intelligence professionals, he says, were much less provocative and certain about the capabilities of Iraq’s leader, Saddam Hussein, and even seemed embarrassed during the briefings when pressed about the possibility of nuclear weapons that could produce the mushroom cloud of the dire administration warnings. Still, with the 2002 election imminent, many Democrats decided they could not take the political risk of a vote against the war. Mr. Feingold describes the party meetings on the war resolution held in a crowded room just off the Senate floor as “among the saddest spectacles I witnessed in 18 years in the Senate.” “People who would later (and not much later) be candidates for president and compete for the anti-Iraq War mantle voiced substantive objections to the war then,” Mr. Feingold recounts. “But when it came time for the vote, Hillary Clinton, John Edwards and John Kerry all rose from their seats and said ‘aye’ to George Bush’s ‘war of choice.’ ” In the Senate, Mr. Feingold had a reputation as a loner willing to break from his party, whether it was on the confirmation of John Ashcroft as attorney general (he supported him) or on a motion to dismiss the impeachment case against President Bill Clinton (he was the only Democrat to oppose it). Mr. Feingold liked sitting in the back row of the Senate, was close to his staff and was one of the few, in a legislative body populated by very wealthy people, who sometimes found himself living paycheck to paycheck. He pictures himself in the vein of Wisconsin figures like Robert M. La Follette and William Proxmire. “I call them as I see them,” said Mr. Feingold, whose book recounts the joke that for a time, his first name could have been McCain because of the high-profile campaign finance law he championed. “I generally vote Democratic, but I simply believe that my job was to be independent and to be ready to work with the other party. That is what the people in my state liked.” His book details a showdown he had with Tom Daschle, then the majority leader, over Mr. Feingold’s vow to filibuster the Patriot Act unless he was allowed to offer amendments. It recalls the difficult conditions on the Hill when senators were forced from their offices by the anthrax attack. There are amusing moments, too, among them when Senator Carl Levin, the grandfatherly Michigan Democrat, approached Mr. Feingold and Paul Wellstone on the Senate floor during the 1999 impeachment trial of Mr. Clinton. “I am a little embarrassed to ask you guys this,” Mr. Levin said sheepishly. “But what’s a thong?” Mr. Feingold, who took a seat on the Foreign Relations Committee despite warnings it was in decline, offers his own warning: that in the time since Sept. 11, America has become too inwardly focused and could again find itself surprised from overseas. “Somehow,” he said, “we have to convince the American people that awareness of the rest of the world is important for our safety.”