US Democrat front-runner John Kerry has strengthened his position, winning the Virginia and Tennessee primaries.
Kerry has now carried 12 out of 14 states
The victories show Mr Kerry, a northerner, can attract voters from the states of the Old South, even running against southern rivals.
Mr Kerry has now won 12 out of 14 state contests to pick the Democrat challenger for the US presidency, and his closest rival, John Edwards, lags well behind.
As the results came in, Wesley Clark decided to pull out of the race.
"This is the end of the campaign for the presidency," Mr Clark said at a news conference on Wednesday in his hometown of Little Rock, Arkansas.
In Virginia, Mr Kerry was backed by more than half the voters with over 90% of the votes counted.
In Tennessee his margin of victory is smaller, but still decisive.
"Americans are voting for change - East, West, North and now in the South," a triumphant Mr Kerry told a large crowd of his supporters in Fairfax, Virginia.
"The message rings out loud and clear," he said.
"People want change in the country. They want to move forward in a new direction and I think I'm articulating what that new direction can be. It's crossing all lines... without regard to region and other labels".
Tennessee is one of the states seen as important to any Democrat hoping to take the White House in November. It voted for Bill Clinton in 1996 but backed Mr Bush over Al Gore, who had served as senator for the state, in the 2000 race.
But the fact that Mr Kerry has shown he has appeal with Democratic voters in the South does not mean he will win there in the general election, says BBC Washington Correspondent Rob Watson.
In 2000, George W Bush made a clean sweep of the South.
Mr Kerry entered the Tuesday primaries boosted by wins in three states which picked their choice of candidate last weekend.
The results from Maine, Michigan and Washington left Mr Kerry riding high.
But Mr Kerry's two losses - in South Carolina to Mr Edwards and a third-place in Oklahoma - came in the south, and so he was eager to win on Tuesday.
After Mr Kerry's latest triumphs, the pressure is likely to build on all his challengers to consider stepping aside so the Democrats can unite behind the Massachusetts senator and against their common enemy President Bush, the BBC's Rob Watson says.
Mr Clark told a crowd of his somewhat subdued supporters in Tennessee: "We may have lost this battle but we are not going to lose the battle for America's future."
But Mr Edwards vowed to fight on, despite the setback.
"We are going to have a campaign and an election, not a coronation," Mr Edwards told his supporters at a rally in Milwaukee, Wisconsin.
Earlier, he said would take his campaign up to and beyond so-called "Super Tuesday" of 2 March when a number of large states, including California and New York, vote for a candidate.
Early front-runner Howard Dean has suggested he will stay in the race even if he does not win the Wisconsin primary on 17 February.
He admitted his new stance was an "obvious contradiction" to earlier statements that a loss would end his candidacy. He said he was responding to supporters' pleas to stay in.
About 75% of delegates to the party convention remain up for grabs after the Wisconsin battle.
Civil rights activist Al Sharpton and Representative Dennis Kucinich are also continuing to fight.
Support from 2,612 delegates is needed to secure the Democratic nomination as presidential candidate.
Virginia and Tennessee send a total of 151 delegates to the party convention.
Most delegates who vote for a candidate at the national presidential nominating convention are allocated according to a candidate's support in state-wide caucuses or polls; some delegates are assigned for party leaders and elected officials to allocate.