Indiana and North Carolina are two of the biggest primary contests left in the Democratic voting calendar in the run-up to the party's national convention in August.
Clinton boosted her campaign for the nomination when she won the Pennsylvania primary on April 22.
While Obama has sought to distance himself from controversial remarks made by his former pastor, support for Clinton in North Carolina has grown, eroding Obama's lead in that state.
Clinton is also leading Obama 49 per cent to 43 per cent in Indiana, according to a Suffolk poll with a margin of error of plus or minus four percentage points.
Clinton has argued that she is the Democrats' best hope winning the White House, in part because she is more popular with the party's working-class base.
The candidates clashed on Monday over Clinton's plan to suspend the federal vehicle fuel tax, with Obama saying that his rival's proposal to suspend the levy over a holiday period was a "stunt".
But in a campaign advertisement aired on Monday, Obama said that Clinton was offering "more of the same old negative politics".
Clinton's advertisement claimed she is "the candidate who is going to fight for working people".
A combined 187 delegates are offered in the two primaries, but due to Democratic party rules the delegates are shared out according to the proportion of the vote received by each nominee.
Each nominee requires 2,025 delegates to secure the nomination to run for the US presidency.
|Democrats are concerned about a protracted |
battle between Clinton and Obama [AFP]
With neither candidate likely to reach 2,025 delegates by the final primary vote in June, the contest is likely to be decided by superdelegates – high-ranking Democrats that can vote for their preferred nominee regardless of primary and caucus results.
From contests held so far, Obama currently holds 1,746 pledged delegates to Clinton’s 1,611, according to MSNBC figures, including superdelegates who have already pledged their allegiance to the candidate of their choice.
The 16-month battle between Clinton and Obama has raised concerns in the Democratic party that it will appear disunited to voters ahead of the November general elections.
While the Democratic race continued on Monday, McCain, who is already assured of the Republican nomination, campaigned in North Carolina.
McCain focused on Obama in particular, saying that he doesn't have national security experience.
"Senator Obama wants to sit down with an Iranian leader who is dedicated to wiping Israel off the map - his words," McCain told reporters on his campaign bus on Monday.
McCain was referring to comments by Obama last year that he would be willing to meet with leaders of countries deemed "rogue states" by the current US administration, such as Iran, North Korea and Cuba.
"Senator Obama has obviously has no national security experience, and therefore that's reflected in his judgment on a number of those issues," McCain said.
Tommy Vietor, a spokesman for Obama, responded by criticising McCain's support for the war in Iraq.