In separate briefings, spokesmen for the White House and State Department cited figures suggesting there will be higher levels of participation in the vote on the constitution than the January elections for the transitional assembly.
White House spokesman Scott McClellan said the agreement on 12 October involving a Sunni Arab party should boost participation. But he declined to give projections on the outcome of the vote.
"It appears that Iraqis are enthusiastic about voting on this draft constitution and expressing their view, whether that would be for or against it," McClellan said.
Approval of the draft constitution would clear the way for Iraq to hold its first national elections for a constitutional government in December.
State Department spokesman Adam Ereli said there are tens of thousands more Iraqi poll workers than in the January elections. He said an increase of registered voters to 15.6 million -- 1.3 million more than in January -- was another sign that democracy was strengthening in Iraq.
"The signs are positive, are good, that there will be broad participation, that the results will reflect the views and the will of the Iraqi people, and that there will be a firm basis for moving forward on the next step, which is an election, another nationwide election on 15 December," Ereli said. "And the infrastructure of democracy is growing, is strong."
To be adopted, the draft constitution must be approved by voters across Iraq. It can be defeated if two-thirds of the voters in at least three of Iraq's governorates vote "no."
A defeat of the constitution would likely mean months more political instability and delay U.S. plans to start pulling out troops. Washington also wants the vote to boost its policy to expand democracy in the Middle East.
Neighboring Iran this summer held a presidential election, which U.S. officials said were not credible because many would-be candidates were denied a chance to participate.
White House Chief of Staff Andrew Card told Radio Farda on 13 October that the Iraq referendum vote could have an impact on Iran.
"I know that the Iranian people are looking with great excitement to see what happens in Iraq because the Iranian people love democracy," Card said. "And they certainly would love to have a chance to have a democracy that wasn't run by a theocracy. And I think that they would like to have a government of, by, and for their people."
The vote takes place at a time when U.S. opinion surveys show President George W. Bush with the lowest approval ratings since he took office.
The U.S. Pew Center for the People and the Press released a survey on 13 October showing that for the first time since the Iraq war began a majority of Americans -- 53 percent -- say the U.S. military effort there is not going well.
(Radio Farda's Parichehr Farzam contributed to this report.)
For RFE/RL's full coverage of the 15 October constitutional referendum in Iraq, see "Iraq Votes: Constitution Referendum"
For weekly news and analysis on Iraq, subscribe to "RFE/RL Iraq Report."