Jack Reed and Chuck Hagel, Democratic and Republican senators, travelled with Obama. All three are critics of the current Bush administration's handling of the war and the subsequent occupation.
Obama in particular, reiterating his campaign promises, has called for the gradual withdrawal of US troops at the rate of one or two brigades a month, as well as an end to combat operations within 16 months.
He has said he favours leaving a residual force in the country to provide security for US personnel, to train Iraqis and counter attacks by al-Qaeda.
'On ground assessment'
Al-Maliki had recently commented in a German magazine endorsing Obama's 16-month timetable.
But his aides said his remarks were misunderstood, and claimed that he is not taking sides in the upcoming US election.
Al-Maliki and George Bush, the US president, have agreed on a so-called "time-horizon" for the withdrawal of US forces as part of a wider security pact still being negotiated.
The Obama campaign has said the aim of his tour is to make an on-the-ground assessment of conditions in the country, as well as meeting leaders, whom he has previously criticised for not doing enough to rebuild Iraq.
"Iraq's leaders have failed to invest tens of billions of dollars in oil revenues in rebuilding their own country, and they have not reached the political accommodation that was the stated purpose of the [US troop] surge," Obama wrote in The New York Times newspaper last week.
John McCain, the rival Republican candidate, is critical of Obama's position on Iraq, and insists that the decision to pull out of the country should be detemrined by progress on the ground, and not a timetable.
|McCain said Obama is 'inexperienced' with respect to US foreign policy [AFP]
Randy Scheunemann, McCain's foreign policy adviser, said Obama "is stubbornly adhering to an unconditional withdrawal that places politics above the advice of our military commanders, the success of our troops, and the security of the American people".
"Barack Obama is wrong to advocate withdrawal at any cost just as he was wrong to oppose the surge that has put victory within reach," Scheunemann said in a statement.
Obama argues that more US troops should be sent to Afghanistan, which he says is the "central front" in the so-called "war on terror". McCain also supports sending troop reinforcements to Afghanistan.
"There's starting to be a growing consensus that it's time for us to withdraw some of our combat troops out of Iraq, deploy them here in Afghanistan, and I think we have to seize that opportunity. Now is the time for us to do it," Obama said after a two-hour meeting with Hamid Karzai, the Afghan president, on Sunday.
"I think it's important for us to begin planning for those brigades now. If we wait until the next administration, it could be a year before we get those additional troops on the ground here in Afghanistan, and I think that would be a mistake. I think the situation is getting urgent enough that we have got to start doing something now."
Obama has made Afghanistan a centrepiece of his proposed strategy for dealing with so-called "terror threats" to the US.
He has said the war in Afghanistan, where the Taliban group has been resurgent, deserves more troops and attention than the conflict in Iraq.
US military officials say the number of attacks in eastern Afghanistan, where most of the foreign troops are American, has increased by 40 per cent in 2008 compared to the same period in 2007.