President Bush says Iraqi voters are striking a blow against terrorists by taking part in Saturday's constitutional referendum. Democrats say the president needs to do more to include more of Iraq's Sunni Arab minority in the political process.

President Bush says Iraq's constitutional referendum is a momentous time in the history of the Middle East.

"By casting their ballots, the Iraqi people deal a severe blow to the terrorists, and send a clear message to the world: Iraqis will decide the future of their country through peaceful elections, not violent insurgency," he said.

In his weekly radio address, President Bush said the referendum is a critical step forward in Iraq's march toward democracy. And with each step of that march, he says, al-Qaida's vision for the region becomes more remote.

Mr. Bush read from a letter the United States says it intercepted between al-Qaida deputy Ayman al-Zawahiri and Jordanian militant Abu Musab al-Zarqawi, who is leading much of the insurgency within Iraq. The president says that letter shows that al-Qaida intends to make Iraq a haven for terrorists and a staging ground for attacks against other nations, including the United States.

"The terrorists know their only chance for success is to break our will and force us to retreat," the president said. "The al-Qaida letter points to Vietnam as a model. Zawahiri says: 'The aftermath of the collapse of American power in Vietnam, and how they ran and left their agents, is noteworthy.' Al-Qaida believes that America can be made to run again. They are gravely mistaken. America will not run, and we will not forget our responsibilities."

In the Democratic radio address, retired Army General Wesley Clark said Iraq's constitutional referendum is an important political event, but, he says, it is not the end of the story.

General Clark, who sought his party's presidential nomination last year, says President Bush has failed to involve more Sunni Arabs in building a domestic political consensus around a new Iraqi government.

"The Sunni minority is isolated, fatalistic, increasingly anti-American," he noted. " Many of the Iraqi forces we are training are able to fight with U.S. leadership and support, but their real loyalty is in doubt. Militia forces are strong. They are well organized. And, in fact, they are the ultimate arbiters in Iraq, and they are preparing to settle factional disputes by force."

General Clark says he agrees with President Bush that it would embolden terrorists, if U.S. troops leave Iraq without enough Iraqi security forces to control their own country. But General Clark says the president needs to be more specific about how many Iraqi units will be needed, before American troops come home.

"We understand what's at stake in Iraq, and that's why we are so frustrated by the president's failure to give the American people clear answers to basic questions. Mr. Bush is long overdue in providing a plan to achieve Iraqi military sufficiency," Gen. Clark said.

A simple majority of voters in Iraq is needed to pass the constitution, but it will fail if two-thirds of voters in at least three provinces reject it. Final results are expected in about three days.