The two main Democratic presidential candidates, Hillary Clinton and Barack Obama, are preparing to face each other in a live televised debate.
The two main Democratic presidential candidates, Hillary Clinton and Barack Obama, have begun a debate by attacking each other's campaign tactics.
It will be their final encounter before next week's crucial primaries in Ohio and Texas.
The two have also been outlining their healthcare policies, with each saying they stand for universal healthcare.
It is their final face-to-face encounter before next week's crucial primaries in Ohio and Texas.
Mr Obama is regarded as the front-runner, after winning the previous 11 primaries and caucuses.
Mr Obama is regarded as the front-runner, after winning the previous 11 primaries and caucuses.
Ahead of the debate he won endorsement from a former rival for the Democratic nomination, Senator Christopher Dodd.
Analysts say Mrs Clinton needs big wins in both states to stay in the race to choose the Democratic candidate.
Mr Dodd, who abandoned his bid last month, is the first Democratic campaign drop-out to endorse another candidate.
Accusations traded
On Tuesday he and Mr Obama appeared at a joint news conference in Cleveland, Ohio, where the debate will be held at 0200 GMT.
The contest between the two Democratic front-runners has become increasingly heated in recent days.
Christopher Dodd is fully behind Barack Obama
Mr Obama's team has accused the Clinton camp of releasing a photo showing Mr Obama wearing traditional Somali robes during a visit to Kenya in 2006.
Mr Dodd said Mr Obama had "been poked and prodded, analysed and criticised, called too green, too trusting and for all of that has already won" more than half the states and millions of votes.
At their debate in Cleveland, Ohio, Mr Obama drew a line under the controversy, saying he believed Mrs Clinton when she said she did not know where the photo had come from.
The BBC's Justin Webb says Mr Obama's campaign is still picking up support and speed.
In a debate last week, Mrs Clinton accused Mr Obama of political plagiarism - a reference to his apparent copying of a supporter's rhetoric.
Analysts say Mrs Clinton needs to win convincingly in both Texas and Ohio to remain in the race to choose the Democratic candidate for November's presidential election.
The race between the two Democratic front-runners has become increasingly heated in recent days.
Last week, during the first of two debates ahead of the 4 March primaries, Mrs Clinton accused Mr Obama of political plagiarism.
She was referring to her rival's apparent copying of rhetoric used by one of his supporters, Massachusetts Governor Deval Patrick.
Mr Obama dismissed the charge as part of the "silly season".
Cheap shots?
On Monday, the two Democratic candidates traded accusations about a photo circulating on the internet showing Mr Obama wearing traditional Somali robes during a visit to Kenya in 2006.
The Drudge Report website said it was circulated by Mrs Clinton's staff but her team denied it had sanctioned its release or that it ridiculed Mr Obama.
Mrs Clinton now needs to win a majority of the delegates in the remaining state primaries and caucuses to stay in the race to choose the Democratic candidate for November's presidential election.
Addressing the issue briefly in an interview with a Texas radio station, Mr Obama said: "I think the American people are saddened when they see these kind of politics."
Ohio and Texas, both big states, are being seen as must wins for her.
Ahead of Tuesday's crucial debate, Mrs Clinton described Mr Obama as untested and stressed her own experience.
Several polls suggest Mr Obama is gaining ground in both Texas and Ohio. The Illinois senator is leading in Texas for the first time, with 50% compared to 46% for Mrs Clinton, according to a CNN poll.
In a speech on Monday she said her rival would need a "foreign policy instruction manual" to keep the country safe if elected.
On Tuesday, Mr Obama won endorsement from a former rival for the Democratic nomination, Senator Christopher Dodd.
Several polls suggest Mr Obama is gaining ground in both Texas and Ohio.
On the Republican side, front-runner John McCain has also been campaigning in Ohio, as the party prepares to hold its own primary in the state, also on 4 March.
According to a CNN poll, the Illinois senator is leading in Texas for the first time, with 50% compared to 46% for Mrs Clinton.
Mr McCain is popular with moderates and independents
On the Republican side, front-runner John McCain has also been campaigning in Ohio.
During a rally in Cincinnati, conservative radio talk-show host Bill Cunningham warmed up a crowd by referring to Mr Obama as "Barack Hussein Obama" three times, emphasising his Muslim-sounding middle name.
Some opponents appear to believe that implying Mr Obama is a Muslim - which he is not - will weaken his appeal to voters.
McCain was not in the building when Mr Cunningham spoke, but came on stage later.
The Republican candidate quickly distanced himself from the attack on Mr Obama, and told reporters: "I apologise for it."
