Wisconsin's Russ Feingold was no ordinary Democratic senator. He staunchly staked out unabashedly liberal positions on all things foreign policy and national security related, right up until his defeat Tuesday night.

Feingold is, or was, technically the third-ranking Democratic senator on the Foreign Relations Committee, after Chairman John Kerry (D-MA) and Chris Dodd (D-CT). With Dodd retiring, Feingold stood to become chairman if Kerry were ever tapped for secretary of state. In fact, the rumor around town is that the prospect of an independent-minded Feingold leading the panel worried the White House so much that it had negative implications on their consideration of Kerry for Foggy Bottom.

Even as a mere rank-and-file committee member, Feingold was more active on foreign policy than most. He had as many as five full-time staffers on the issues, we're told, which is more than double the contingent for the average senator. Feingold had an extensive foreign-policy agenda, the leading item of which was his call for the administration to set a flexible timetable for withdrawing troops from Afghanistan.

Feingold emerged after 9/11 as a champion of the liberal opposition to President George W. Bush's policies regarding the global war on terror. He was the only senator to vote against the Patriot Act in 2001 when it first came up for a vote. He voted against giving Bush authorization to wage war in Iraq and pushed for withdrawal timelines throughout the war, often ignoring the wishes of Senate Democratic leadership. He introduced a resolution to censure Bush for violating Americans' civil rights through what he said was illegal domestic wiretapping.

Over the past two years, Feingold pleaded with the Obama administration to fulfill its promise to close the prison at Guantánamo Bay, Cuba. He also used his perch as chair of the Africa affairs subcommittee to call for changes in U.S. policy toward Sudan. Whereas the House has a caucus of dozens of liberal antiwar lawmakers, in the Senate, Feingold led the few who shared his views and made sure those views entered the public debate.

On the economic front, Feingold resisted all free-trade agreements as well as Obama's efforts to relax export controls to countries like China and India. Those in the foreign-policy community who agree with those positions just lost their greatest advocate on Capitol Hill.

Notably, Feingold was also genuinely committed to bipartisanship. He famously voted for Attorney General John Ashcroft and voted against Treasury Secretary Timothy Geithner only days after Obama's inauguration. He also worked with John McCain to craft campaign finance-reform legislation.

With Feingold gone, the Senate Foreign Relations Committee's Democratic roster is more centrist, just as the Republican side of the bench is set to become more conservative. With Dodd also leaving the Senate this year, that's a lot of institutional knowledge to lose in one night.

And if Kerry ever does become secretary of state, Feingold is no longer in the running to replace him. Kerry's departure would leave the job of chairman of the Senate Foreign Relations Committee up for grabs, with Robert Menendez (D-NJ) and Barbara Boxer (D-CA) as the early favorites.