The US Senate on Tuesday lost a champion of human rights in Africa and defender of civil liberties at home as Wisconsin voters defeated Democratic Senator Russell Feingold's bid for reelection.

Feingold, the first sitting US senator to call for a firm timetable for withdrawing US forces from Afghanistan, fell to Republican businessman Ron Johnson in an election shaped by deep voter anger at the sour economy.

Johnson, who poured millions of his own money into the campaign, was quickly declared the winner and had won 53 percent of the vote with 88 percent of precincts reporting.

"America is at a tipping point and tonight the people of Wisconsin have sent a very strong message," Johnson said after thanking Feingold for his service.

Johnson vowed to take his responsibilities "seriously," attack "reckless spending" in Washington and "get America back to work."

"Washington is too powerful and it is taking us in the wrong direction," he said in a statement.

Feingold thanked Wisconsinites for the "great privilege" of serving as their senator.

"So it's on to the next fight," he told cheering supporters. "It's on to 2012 and it's on to our next adventure forward."

First elected to the Senate in 1992, Feingold had also authored, with Republican Senator John McCain, legislation to curb the flood of money in US campaigns, and was a staunch foe of the death penalty.

Feingold had been a thorn in the side of President Barack Obama and his Republican predecessor, George W. Bush, as they sought ever-wider surveillance and detention authorities to fight the global war on terrorism.

"Feingold is the only true champion of civil liberties in the US Senate," said constitutional lawyer Glenn Greenwald, a fierce critic of the risk posed to US constitutional protections by expanding government power.

Greenwald noted that Feingold had stood up to his own party over the issue and deplored: "Both political principle and the US Constitution have suffered a genuine blow from the loss of this irreplaceable, stalwart advocate."

The senator, who will leave office come January when a new Congress is sworn in, also served as chairman of the Senate foreign relations subcommittee on African affairs and pushed for greater US engagement with the continent.

"Senator Feingold will be sorely missed. He follows the issues closely and it shows," said Sam Bell, executive director of the Save Darfur Coalition / Genocide Intervention Network, who praised the lawmaker's advocacy for a "comprehensive" US policy in Sudan.

Bell also noted Feingold's support for a robust US response to the brutal rebellion by the Lord's Resistance Army in central Africa.

Feingold was among the minority of senators who voted against giving Bush the authority to invade Iraq, and was the first sitting senator to call for a timetable for withdrawal from that strife-torn country.

In the run up to Obama's announcement that he would send some 30,000 more troops to Afghanistan, Feingold warned that he was "not convinced that simply pouring more and more troops" there was "a well thought out strategy."

Feingold's independent streak was often on display: he was the only Democratic senator to vote in 1999 against dismissing the sex-and-lies impeachment trial against then-president Bill Clinton before it began.

(Source: AFP)