By Charles Babington and Jonathan Weisman
Washington Post Staff Writers
Thursday, December 14, 2006; A01

Sen. Tim Johnson (D-S.D.) was in surgery last night after falling ill at the Capitol, introducing a note of uncertainty over control of the Senate just weeks before Democrats are to take over with a one-vote margin.

Johnson, 59, was taken to George Washington University Hospital shortly after noon, where he underwent "a comprehensive evaluation by the stroke team," his office said. Aides later said he had not suffered a stroke or heart attack, but they offered no further comment or details of the surgery.

The two-term senator's illness -- which sent Senate Democratic leader Harry M. Reid (Nev.) rushing to the hospital to check on Johnson -- underscored the fragility of Democrats' hold on the next Senate, which they won by the narrowest of margins in the Nov. 7 elections. Should Johnson be unable to complete his term, South Dakota's Republican governor, Michael Rounds, would name a replacement for the next two years.

With Johnson in office, Democrats would hold a 51-to-49 edge in the Senate that convenes Jan. 4 as part of the 110th Congress. (The two independents have said they will caucus with the Democrats.) But if he is to leave office before then and Rounds replaces him with a Republican, the GOP would control the chamber.

In a 50-50 Senate, Vice President Cheney could break tie votes in the GOP's favor. But a Senate that becomes evenly split after it is in session would not necessarily fall to Republicans, Senate historians said. Rules and precedents could leave a party in charge of the chamber even after its membership falls below that of the other party.

"It's what happens in January that counts," said Senate associate historian Donald A. Ritchie, referring to when party leaders hash out rules governing the chamber's organization.

Rounds's office declined to comment on the situation yesterday except for a statement from the governor, which offered prayers for Johnson and hope for "good news for our friend and colleague."

Johnson spokesman Noah Pinegar said the senator "became disoriented" during a late-morning conference call with reporters, placed from the Capitol's Senate recording studio. "He had difficulty completing a response to a question," Pinegar said, so aides ended the call and walked with him back to Hart Senate Office Building.

When they arrived, Pinegar said, Johnson "wasn't himself." A team from the Capitol physician's office quickly arrived and sent the senator to the hospital by ambulance. Johnson's wife, Barbara, was with him at the hospital as tests were being conducted last night, Pinegar said.

Reid spent much of the afternoon and evening with Johnson's family at the hospital, said spokesman Jim Manley. He would not comment on Johnson's condition.

The Sioux Falls Argus Leader's Web site said that Rep. Stephanie Herseth (D-S.D.) asked constituents to pray for Johnson and that she said she thought Johnson had suffered a severe stroke. But that was before Johnson's staff had ruled out a stroke.

The only time that partisan control of the Senate changed in mid-session, historians say, was in 2001. Republicans began the year controlling the 50-50 chamber with Cheney's tie-breaking vote. But Democrats, mindful of the recent sudden death of Sen. Paul Coverdell (R-Ga.), were aware they could be a heartbeat away from the majority.

In order to adopt new rules organizing the Senate, the two parties must reach nearly unanimous agreement. Democrats in 2001 blocked the naming of committee chairmen and members, demanding concessions before agreeing to the rules. Among those concessions: Should the numerical advantage change, all committee assignments and chairmanships would be nullified, and a new organization would have to be submitted.

That's what happened, not because of a death but because disgruntled moderate Republican Sen. James M. Jeffords (Vt.) decided to caucus with the Democrats, giving them a 51-49 edge and the powers of the majority. Senate Republican sources said yesterday that their party is likely to press for similar concessions when negotiating the operating rules for the next Congress. But even if Johnson were incapacitated, Democratic aides say, they would resist.

A different scenario unfolded in 1954, after the deaths and replacements of several senators over two years. Republicans remained the majority party even though Democrats eventually outnumbered them, 48 to 47, with one independent. Democratic leader Lyndon B. Johnson did not challenge the GOP's control, in part, historians said, because the independent, Wayne L. Morse of Oregon, warned that he would caucus with the Republicans if need be. That would have led to a 48-48 chamber, and Vice President Richard M. Nixon would have broken the tie in Republicans' favor.

Tim Johnson has a quiet demeanor and low profile in Washington, but he won two impressive Senate victories. As a House member in 1996, he ousted GOP Sen. Larry Pressler, then chairman of the Commerce Committee.

Six years later, Johnson managed a 524-vote win over Republican John Thune. Thune returned two years later to defeat Thomas A. Daschle, the Senate Democratic leader, in a costly, closely watched election.

Thune had attacked Johnson for voting against the Persian Gulf War resolution in January 1991. When Congress in 2002 debated authorizing Bush to invade Iraq, Johnson announced he would vote aye.

In a floor speech, he said: "There is a strong possibility that I may be voting to send my own son into combat, and that gives me special empathy for the families of other American service men and women whose own sons and daughters may also be sent to Iraq. Nevertheless, I am willing to cast this vote -- one of the most important in my career both as a senator and certainly as a father -- because I recognize the threat that Saddam Hussein represents to world peace."

The senator's son, Staff Sgt. Brooks Johnson, was stationed in Afghanistan and Iraq before becoming an Army recruiter in Illinois.

Given Johnson's narrow reelection win over Thune, and South Dakota's GOP tilt -- Bush carried the state by 22 percentage points in 2004 -- Republicans consider Johnson one of their top targets in the 2008 Senate elections.

The leading candidate to oppose him is Rounds, who was reelected to a second term last month. Johnson had previously committed to seeking a third term. Should he not run, Democrats probably would turn first to Herseth, who easily won reelection last month.

Staff writers Lyndsey Layton and Allan Lengel, political researcher Zachary A. Goldfarb, staff researcher Madonna Lebling and washingtonpost.com staff writer Chris Cillizza contributed to this report.