The Senate majority will turn on razor-thin races in Virginia and Montana, where recounts or legal challenges could delay the final outcome for days. Democrats moved within striking distance by ousting Republicans in Ohio, Pennsylvania, Rhode Island and Missouri.

Republicans, facing voter hostility toward the Iraq war and the Bush administration, had hoped that the worst outcome would be a 50 to 50 Senate, allowing Vice President Dick Cheney to break tie votes. With such a narrow margin, Senate Republicans probably would face difficult battles with the first Democratic-controlled House in a dozen years.

In every section of the country, Democrats attacked Republican incumbents for their links to President Bush, whose handling of Iraq was deeply unpopular with millions of voters. The war, coupled with Republican scandals, proved lethal to at least three GOP senators, including the party's third-ranking senator, Rick Santorum of Pennsylvania. A sharp-tongued conservative and Bush loyalist, Santorum fell to Democrat Robert P. Casey Jr., the state treasurer and son of a popular former governor.

Another two-term Republican, Mike DeWine of Ohio, was hurt by events largely beyond his control, namely a string of scandals involving Ohio Republicans. Democratic Rep. Sherrod Brown, who touted his vote against the Iraq invasion, successfully hammered DeWine on his ties to the administration.

In heavily Democratic Rhode Island, GOP Sen. Lincoln Chafee stressed his independence from Bush and attacked the record of Democratic nominee Sheldon Whitehouse, a former state attorney general. But even his family's famous name could not save Chafee this time.

Missouri's Senate race had been close for weeks, with Democratic challenger Claire McCaskill attacking first-term Sen. James M. Talent's ties to Bush. McCaskill, the state auditor, came out on top after campaigning feverishly in Missouri's rural areas and small towns, where Democrats have faltered in past elections.

Republican hopes of grabbing Democratic seats in Maryland and New Jersey faltered, as spirited and well-funded campaigns appeared to fall short in states that traditionally are not friendly GOP turf. But Republicans won a tough contest in Tennessee.

And three-term Sen. Joseph I. Lieberman of Connecticut, who lost the Democratic nomination to antiwar candidate Ned Lamont in August, was elected as an independent. He will caucus with the Democrats.

Democrats needed six new seats to take over the Senate, where the GOP's 55 to 45 advantage was built largely on victories in the South two years ago. This year, many of the toughest contests were in the Northeast and in heartland areas such as Ohio and Missouri.

The Democratic strategy of criticizing Bush's war strategy — without necessarily offering an alternative — was effective, according to nationwide surveys of voters exiting polling places. Clear majorities of respondents said the Iraq war was important or extremely important to them, and most of them backed Democratic candidates.

The exit polls also suggested that public corruption may have played a bigger role in the election than many strategists expected. That helped Democrats in a year when scandals involving lobbyist Jack Abramoff and former GOP representatives Tom DeLay of Texas and Mark Foley of Florida made headlines for weeks.

Some Republicans caused their own problems. GOP Sen. George Allen injured himself by publicly taunting a Democratic campaign worker who is Indian American and by awkwardly handling news of his Jewish ancestry. Democrat James Webb, a former Reagan administration official, remained locked in a tight battle with Allen.

In Montana, scandals and gaffes hurt Republican Sen. Conrad Burns during the summer, and he spent the fall trying to catch Democrat Jon Tester. Burns accepted contributions from — and aided the clients of — Abramoff, the disgraced lobbyist. He also publicly criticized out-of-state firefighters who had come to Montana's aid. Tester, president of the state Senate, said the three-term incumbent was out of touch with his constituents after 18 years in Washington.

The Tennessee race to replace departing Senate Republican Majority Leader Bill Frist drew national attention, in part because of controversial ads aimed at Democratic nominee Harold E. Ford Jr., who is black. Former Chattanooga mayor Bob Corker said Ford, a five-term House member from Memphis, was too liberal for the state, and won a narrow victory.

In New Jersey, Democratic Sen. Robert Menendez held off a strong challenge by Republican Thomas H. Kean Jr., who had accused him of having of ties to corrupt state officials and contractors.

In Maryland, the GOP hoped to capture the seat of retiring Democratic Sen. Paul Sarbanes by nominating a high-profile African American, Lt. Gov. Michael S. Steele. But Democratic Rep. Benjamin Cardin's by-the-books campaign, which stressed his 20 years in the House, made him the winner.

In Minnesota, Republicans once had high hopes of grabbing the seat being vacated by Democratic Sen. Mark Dayton. But GOP Rep. Mark Kennedy fell to Democrat Amy Klobuchar, the Hennepin County attorney.

The Senate will have two independent members who will caucus with the Democrats: Lieberman and Rep. Bernard Sanders of Vermont, a self-described socialist who succeeded the retiring Sen. James Jeffords, also an independent. In New York, Democratic Sen. Hillary Rodham Clinton breezed past GOP challenger John Spencer, positioning her for what many believe will be a 2008 presidential bid.

The GOP failed to recruit strong candidates in several Republican-leaning states, resulting in easy re-elections for Democrats Kent Conrad of North Dakota and Ben Nelson of Nebraska. Sen. Bill Nelson, D-Fla., handily defeated mistake-prone Rep. Katherine Harris (R), who had played a key role in the president's contested win in Florida in 2000. Republicans once hoped to oust Michigan Democratic Sen. Debbie Stabenow, but she defeated Republican Michael Bouchard.

In Arizona, GOP Sen. Jon Kyl survived a challenge from former Democratic Party official Jim Pederson. In Nevada, GOP Sen. John Ensign defeated Democrat Jack Carter, son of former president Jimmy Carter. And in Washington state, Democratic Sen. Maria Cantwell fended off Republican insurance executive Mike McGavick.