US President George W Bush has robustly defended his new Iraq policy, telling a TV interviewer that congressional opponents would not weaken his resolve.
Mr Bush gave his interview from his Camp David retreat
Mr Bush told CBS show 60 Minutes that failure in Iraq would strengthen Iran and pose a threat to world peace.
Both houses of Congress, now controlled by Democrats, say they will vote on the plan to send more troops into Iraq.
Vice-President Dick Cheney hit out at opponents, saying Congress would not succeed in running "war by committee".
Mr Cheney became the latest senior member of the Bush administration to explicitly criticise Iran for allegedly destabilising the situation in Iraq.
He urged the Iranians to "keep their folks at home", referring to US suspicions that Iran's Revolutionary Guard is active in Iraq training and arming Shia militants.
In his CBS interview, broadcast at primetime on Sunday evening, the president also took a tough line on Iran, and said he had considered all options before opting to send more troops to Iraq.
Those options included beginning a withdrawal, Mr Bush said.
"I've thought about that, and my attitude is if we were to start withdrawing now, we'd have a crisis in our hands in Iraq.
"And not only in Iraq, but failure in Iraq will embolden the enemy. And the enemy is al-Qaeda and extremists. Failure in Iraq would empower Iran, which poses a significant threat to world peace. "
Mr Cheney offered strong support to Mr Bush, praising his determination over Iraq.
"He's the one who has to make these tough decisions. He's the guy who's got to decide how to use the force and where to deploy the force.
Mr Cheney, who has stayed out of the limelight in recent months, criticised congressional opponents - either Democrats or Republicans - who he said had not yet offered any coherent alternative policy on Iraq.
The biggest boost to enemies of the US, Mr Cheney said, would be to give up because the war had become too difficult.
"They're convinced that the United States will pack it in and go if they just kill enough of us."
Mr Bush and Mr Cheney joined Secretary of State Condoleezza Rice and National Security Adviser Stephen Hadley in speaking out publicly against Iran.
Last week US forces detained several Iranians in northern Iraq on suspicion of aiding insurgents, accusations rejected by Tehran.
In Iraq, US casualty numbers continue to rise each day
But the Iraqi Foreign Minister Hoshyar Zebari warned that growing tensions between Iran and the US have left Iraq treading a "thin line".
"The Iraqi government has national interests of its own," Mr Zebari said.
"We can't change the geographical reality that Iran is our neighbour. This is a delicate balance and we are treading a very thin line."
As well as public pronouncements, senior officials are already holding talks with allies on the new policy.
Defence Secretary Robert Gates is in the UK for talks on Afghanistan and Iraq, while Ms Rice is part-way through a week-long diplomatic tour of the Middle East.