President Barack Obama's conservative critics on Friday blasted his announcement that all US troops will leave Iraq by year's end, citing worries US archnemesis Iran will fill the vacuum.

Republican Senator John McCain, Obama's rival for the White House in 2008, described the news as a "consequential failure" for Obama as well as Iraq's leaders after talks broke down on leaving a small US force beyond December 31.

"This decision will be viewed as a strategic victory for our enemies in the Middle East, especially the Iranian regime, which has worked relentlessly to ensure a full withdrawal of US troops from Iraq," he said.

Frederick Kagan, widely seen as the intellectual architect of the 2007 US troop "surge" credited in Washington with pulling Iraq from the brink of civil war, echoed the charge.

"The withdrawal of American military protection from a state helpless to defend itself on its own effectively throws Iraq into the arms of Iran, however the Iraqis feel about the matter," he wrote in a blog post.

Talks on extending the US presence broke down because the two sides were unable to agree on granting legal immunity for American troops who would have stayed in place to help train Iraqi forces and to counter Iran.

Republicans have watched worriedly as top Iraqi officials and their Iranian counterparts have stepped up diplomatic contacts in the wake of the 2003 US-led invasion to topple the late dictator Saddam Hussein.

Democrats have argued it was the invasion itself that ended Iraq's role as a historic bulwark against Iran, with which it fought a devastating 1980-88 war, and noted that the December 2011 withdrawal date was central to a US-Iraq accord signed in 2008 under Republican president George W. Bush.

And they emphasized that Iraq, where some leading political leaders have expressed anger about Iranian "meddling" in the strife-torn country's internal affairs, is a sovereign state.

"The United States is fulfilling our agreement with an Iraqi government that wants to shape its own future," said Senate Foreign Relations Committee Chairman John Kerry, a Democrat.

"I fully support the president," Democratic Senate Majority Leader Harry Reid told reporters on a conference call in which he also played down fears that the US withdrawal would embolden Tehran.

Reid -- who drew fire in April 2007 for saying of Iraq that "this war is lost" -- said the United States was "keeping a real close eye on Iran" and would be "well equipped" to do so even after the withdrawal.

Iran is "way behind (the) times," he said, warning leaders in Tehran that the storm of "Arab Spring" pro-democracy movements that have swept aside authoritarian regimes in the Middle East this year would soon reach them.

"They should be aware that the spring that hit all of that part of the world is about to hit them," he said.

Republican House Speaker John Boehner offered a measured appraisal that urged Obama to "continue engaging with the Iraqi government in a way that ensures our hard-fought gains translate into long-term success."

"While I'm concerned that a full withdrawal could jeopardize those gains, I'm hopeful that both countries will work together to guarantee that a free and democratic Iraq remains a strong and stable partner for the United States in the Middle East," Boehner said in a statement.

And Obama got an unexpected vote of confidence from former Bush press secretary Ari Fleischer, who said on his Twitter feed that the "decision on Iraq is right one."

"I was open 2 staying if he made the case it wld help w Iran, but Iraq war is over. It's time," said Fleischer.

But Republican presidential candidate Mitt Romney, a former Massachusetts governor, angrily assailed Obama's "astonishing failure" to keep a residual US force in Iraq.

"The unavoidable question is whether this decision is the result of a naked political calculation or simply sheer ineptitude in negotiations with the Iraqi government," he said.

The issue was unlikely to shape Obama's bid for a second term in November 2012 elections: Poll after poll of public opinion has found barely five percent of Americans view the wars in Iraq and Afghanistan as their number one issue.