In the 22-nation Arab bloc, the distinct prospect of a Shiite-dominated government in Iraq causes the deepest anxiety among Sunni Muslims, who have largely dominated Middle East societies since the faith was founded 15 centuries ago. Shiites have been a largely disenfranchised underclass even where they are the majority -- as in Iraq, Bahrain and Lebanon.
Several Sunni governments have expressed concern to the United States that a Shiite government -- the first in the Arab world -- could inspire the rise of a new Shiite "crescent" stretching from Iran on the Persian Gulf to Lebanon on the Mediterranean, U.S. officials said.
For some in the region, the balloting is a worrisome prospect whether it succeeds or fails.
If the voting is deemed credible, some fear it will embolden those who say "send in the Marines and change regimes and redraw the map," said Rami Khouri, editor of Lebanon's Daily Star.
And if the elections are judged a failure because of violence or other factors, some fear that outcome would create a sense that extremists have the upper hand, fueling insurgent movements not just in Iraq but elsewhere in the region, analysts said.
"For people in this region, the reality has dawned as election day nears that there is no good outcome. Every outcome has serious consequences, and every outcome is pretty scary," Khouri said.
The Persian Gulf sheikdoms feel particularly vulnerable. Saudi Arabia is estimated to have more than 1 million Shiites, most of whom live in the oil-rich eastern province. Saudi Shiites were involved in a series of rare anti-government protests in the early 1980s -- after predominantly-Shiite Iran's 1979 revolution inspired Shiite communities elsewhere -- because the oil revenue from their region mainly benefited the Sunni population.
"At the core of Saudi concern is this prejudice against the Shia they never enunciate in a policy. They just cite the Iranian bogeyman," said an administration official, referring to Iran's Shiite theocracy. "That's not something we see happening."
As a result, the Bush administration has been conducting a lot of diplomatic hand-holding in recent weeks to allay fears -- and to point out the realities of change. In telephone calls and meetings in Washington and the region, senior administration officials have tried to convince Arab and Turkish leaders particularly that the region is headed for change -- but not chaos.
"This is the tide of history," said the administration official, who spoke on the condition of anonymity because of the sensitive diplomacy involved. "This is recognition of a reality that has existed for a long time and only been denied by extreme violence. So get used to it."