Barack Obama scored a resounding win in the Democratic presidential primary in North Carolina, while Hillary Clinton narrowly won the Indiana primary.
In North Carolina, Senator Obama secured 56 per cent of the vote against 42 per cent for Senator Clinton.
Mrs Clinton, meanwhile, won 51 per cent of the vote in Indiana, compared to Mr Obama's 49 per cent.
Political analysts now believe Mr Obama's lead is looking increasingly unassailable as it looks virtually impossible for Mrs Clinton to catch Mr Obama in pledged delegates.
Tim Russert, the doyen of American political broadcast journalists, said: "We now know who the Democratic nominee will be."
Mr Obama currently leads the fight for delegates with 1,840 to Mrs Clinton's 1,684, according to the Associated Press news agency.
The successful candidate needs 2,025 delegates to secure the Democratic nomination.
Speaking after his comfortable North Carolina primary win, Mr Obama told a rally in Raleigh: "Tonight we stand less than 200 delegates away from securing the Democratic nomination for president of the United States."
Mr Obama's North Carolina win was a much-needed boost for the Illinois senator after losing three consecutive primaries in Ohio, Texas and Pennsylvania.
Mrs Clinton, senator for New York, told cheering supporters at a rally in the Indiana state capital, Indianapolis: "Tonight it's your victory. Tonight we've come from behind. We've broken the tie and thanks to you, it's full speed on to the White House.''
According to the polls, Mr Obama secured the backing of 90 per cent of African-American voters in North Carolina, while Mrs Clinton won 58 per cent of non-black voters in the state.
African-American's make up more than a third of the North Carolina electorate.
Six state primary contests remain - West Virginia, Oregon, Kentucky, Puerto Rico, Montana and South Dakota.
comments
What do you think? Give us your opinion on the comments page.