He was a minority of one four years ago, when the Wisconsin Democrat cast the lone Senate vote against the USA Patriot Act in the traumatic weeks after the Sept. 11 attacks. The law, he said then, gave government too much power to investigate its citizens. Ninety-nine senators disagreed.
Now add more than two dozen senators to Feingold's side, including the leaders of his party and some of the chamber's most conservative Republicans, and the balance of power shifts.
The new Senate arithmetic that emerged this week is enough to place the renewal of major portions of the law in doubt. It was enough to inspire Senate Republican leaders to consider a backup plan in case Feingold's filibuster threat succeeded. Enough to prompt President Bush to dispatch Attorney General Alberto Gonzales to Capitol Hill twice in two days to lobby on the accord's behalf.
No luck so far, said the chief Senate sponsor.
"We've got a battle on our hands," Judiciary Committee Chairman Arlen Specter, R-Pa., told reporters after Gonzales had departed Wednesday.
Bush weighed in personally Thursday, urging opponents of the renewal to abandon the filibuster threats.
"That is a bad decision for the security of the United States," the president said. "I call upon the Senate to end the filibuster and to pass this important legislation so that we have the tools necessary to defend the country in a time of war."
Moments later, the senior Democrat on the issue, Sen. Patrick Leahy, D-Vt., told reporters that more than 40 votes exist to sustain a filibuster in a test vote Friday. White House allies said they would rather see the law's 16 temporary provisions expire entirely than give opponents another three months or more to keep whittling away at them.
Feingold finds himself with some unlikely allies, including the Christian Defense Coalition. Notably, the National Rifle Association has not endorsed the Patriot Act renewal that was personally negotiated by Vice President Dick Cheney. The NRA's non-position allows its supporters in the Senate to oppose renewing the law in its entirety.
"Folks, when we're dealing with civil liberties, you don't compromise them," said Sen. Larry Craig, R-Idaho, an NRA board member.
The breadth of support gives Feingold, a possible presidential candidate, new reason to keep an eye on still other numbers: polls for the 2008 presidential election.
"It's just very gratifying," Feingold said, grinning during an interview this week in his office. "We've stood the test of time. Our concerns were legitimate."
The opposition that began with Feingold's one vote has bloomed into a bloc of Democrats and Republicans concerned about a range of powers the original act gave the FBI, and how they are used. This group prefers the curbs on government power passed by the Senate but rejected in a compromise with the House. Now, faced with an up-or-down vote on the accord, they say no.
Chief among their concerns are the National Security Letters that the FBI can use to compel the release of such private records as financial, computer and library transactions. The bill for the first time explicitly says the third-party recipients of NSLs — banks, Internet service providers and libraries — can hire lawyers and challenge the letters in court.
Feingold and his allies want more reports from the Justice Department on how NSLs and other tools in terror investigations are used. They also want to set limits on how long law enforcement officials can continue to use NSLs in terror investigations.
In the last week, Feingold has attracted important allies, including Senate Minority Leader Harry Reid, D-Nev., and Sen. Chuck Hagel, R-Neb., a possible presidential candidate in 2008.
On Thursday he added another to his column: Sen. Dianne Feinstein of California, the original Democratic co-sponsor of the 2001 Patriot Act.
Whatever happens with the renewal, the mere debate is a boost for Feingold and any presidential aspirations he may nurture after next year's midterm elections — a development that carries some irony.
"People don't go to the well of the Senate and become the only senator to vote against something called the 'USA Patriot Act' five weeks after 9/11 because they're trying to get ready to run for president," Feingold said.
But four years later, during visits to the presidential proving grounds of New Hampshire and Iowa, Feingold says there's evidence his position has resonated with more than just the Democratic base.
"It's something that people like about me," he said. "We'll see where it goes."