The next major tests for the U.S. presidential contenders come in Saturday's Democratic primary in South Carolina and next Tuesday's Republican primary showdown in Florida. VOA National correspondent Jim Malone has the latest.
The latest polls show Senator Barack Obama leading Senator Hillary Clinton in South Carolina, where about half the Democratic voters are African-American.
Obama is hoping for a victory to blunt Clinton's momentum after her recent victories in Nevada and New Hampshire.
Some Democrats are concerned that the negative tone of the Clinton-Obama battle could hurt their chances of winning the White House in November.
The candidates have sought to ease the tension in recent days. Both campaigns withdrew critical radio ads shortly after they began airing.
But it remains clear that both Clinton and Obama are ready to respond in kind if attacked.
"I try not to attack first," said Hillary Clinton on NBC's Today program. "But I have to defend myself and I do have to counterpunch."
Obama told voters in South Carolina that he will respond to attacks from both Hillary and former President Bill Clinton.
"If people are making false assertions about my record, we will answer them," he said.
The Clintons were also a major topic of discussion at a Republican candidates' debate in Florida late Thursday.
Former Massachusetts governor Mitt Romney said he was eager to run against the Clintons in the general election.
"The idea of Bill Clinton back in the White House with nothing do is something I just cannot imagine," he said. "I cannot imagine it is something the American people can imagine."
Polls suggest Romney is in a tight battle in Tuesday's Florida primary with Senator John McCain, who is trying to build momentum after last week's victory in the South Carolina Republican primary.
Former New York City mayor Rudy Giuliani and former Arkansas Governor Mike Huckabee are trailing in Florida. A poor showing could be especially damaging to Giuliani, who chose not to compete in most of the early primaries and caucuses so he could focus on Florida.
Some experts believe the struggles of Giuliani and Huckabee are good news for Romney and McCain.
Norman Ornstein, a political expert at the American Enterprise Institute in Washington, says Romney's use of his vast personal fortune gives him an advantage, and he predicts an eventual showdown between Romney and McCain for the Republican presidential nomination.
"The ability of Romney to finance his own campaign puts him in a strong position, even with a couple of hits [losses] in some of these races, and it is conceivable that we could get this down to a two-person race between the two of them, which would be an interesting prospect," he said.
After the contests in South Carolina and Florida, candidates in both parties will prepare for so-called Super Tuesday on February 5 when more than 20 states will hold primary or caucus votes.