While most world leaders held back from a formal reaction pending the final result of the race for the US Senate -- and speaking before the resignation of Defence Secretary Donald Rumsfeld -- there was general acknowledgement that Iraq had been the main factor behind the Democrats ousting Bush's ruling party from power in the House of Representatives.
Staunch backers of the US invasion of Iraq such as Japan insisted the election results would have no impact on their stance.
"Japan's support for Iraq will not change," Prime Minister Shinzo Abe told reporters in Tokyo.
"The international community has worked to rebuild Iraq. Japan will continue to do what we can," said Abe, who took office in September.
Japanese opposition leaders, however, saw the verdict of the US voters as a major slap in the face for Bush and warned Abe against pursuing what they described as the slavish support of his predecessor, Junichiro Koizumi, for the US president.
"President Bush, who was an ally of former prime minister Koizumi, has been severely judged by his people," said Mizuho Fukushima, head of the liberal Social Democratic Party.
Italian Prime Minister Romano Prodi, who had labelled the Iraq war a "grave mistake" following his election earlier this year, said the conflict was clearly responsible for the Republicans' defeat.
"There were also a few problems in domestic politics, but these also came from the war in Iraq," Prodi said.
If the Republicans also lost control of the Senate, Bush "will certainly be a president who will have to negotiate everything with the opposition," he added.
Danish Prime Minister Anders Fogh Rasmussen, one of Washington's staunchest allies, acknowledged that the Iraqi issue had a "dominant influence" in the result, and called on Bush and the new Congress to work together on key foreign policy issues.
"The world needs a determined and energetic United States," Rasmussen said, adding that he hoped "the president and Congress, under these new conditions, reach a common line on Iraq and Afghanistan."
In Germany, a government spokesman rejected suggestions that the Democrat gains would cripple Bush's foreign policy for the remaining two years of the president's term.
"We do not see the US government's capacity to act on foreign policy affected by the outcome of the elections," spokesman Ulrich Wilhelm told a press conference.
"Regardless of the outcome of the elections, the German government will continue to work with the US government and the US president," he added.
But Karsten Voigt, a key advisor to Chancellor Angela Merkel, predicted that Washington would be forced to rely more heavily on Europe for help with its foreign policy challenges.
"The Democrats have criticised President Bush for not listening enough to the Europeans, to the allies," Voigt said. "The president has already moved in this direction, especially towards the Europeans, but the Democrats will demand that he goes further."
The second largest bloc in the European Parliament, the Socialist Group, sent a congratulatory message to the Democrats, hailing their victory as "the beginning of the end of a six-year nightmare for the world."
Financial markets gave a jittery response to the election result, with European stock exchanges slipping back on concerns that a split in power in Washington would create legislative gridlock in the world's biggest economy.
Turkish Prime Minister Recep Tayyip Erdogan said the election had gone much as expected, even if the Democrat gains were had been higher than initially estimated.
"The Democrats won a very big victory," Erdogan said.
In Iraq itself, many Sunni Arabs gave a guarded reaction to the power shift in Washington, amid fears it might hasten a US military pullout.
"Any withdrawal of the American army would be a big disaster. There would be at the very least a civil war," said Qais Abu Ahmed, a middle-aged Sunni employee in the housing ministry.

Copyright AFP (Agence France-Presse), 2006