BAGHDAD  The Iraqi government is pressing ahead with plans to hold a national referendum on the Iraqi-American security agreement  a measure likely to lose if put to a popular vote with the outcome that American troops could be forced to leave as early as next summer, nearly a year and half ahead of schedule.

Under the security plan agreed to by the two governments last year, American combat troops must withdraw from the cities by the end of this month and all American troops must be out of Iraq by Dec. 31, 2011.

Passage of the agreement was contingent on the approval of several other measures, including legislation requiring a referendum on the agreement. If the Iraqi people vote down the security pact, the American military would have to withdraw all troops within a year from the date of the vote, which could be held as soon as this summer.

American diplomats are quietly lobbying the government not to hold the referendum, but so far Iraqi politicians have decided to go ahead with it to avoid appearing to be in the pocket of the Americans in an election year.