BAGHDAD (Reuters) - U.S. generals expect to need a large contingent of troops in Iraq until the middle of 2009, the commander of U.S. forces in Iraq said on Monday.

Such a timeline would hand President George W. Bush's successor the task of bringing U.S. forces home from Iraq, more than six years after Bush dispatched them to topple Saddam Hussein.

The next U.S. president will take office in January 2009 after an election in 2008.

Bush's Democratic opponents in Congress want U.S. troops in Iraq, which currently number about 157,000, to leave sooner.

Asked about media reports that Washington envisioned a substantial American force remaining in Iraq through mid-2009, General David Petraeus told ABC News: "Sustainable security is, in fact, what we hope to achieve.

"It's in our campaign plan. We do think it will take about that amount of time, as you discussed, to establish the conditions for it."

Petraeus said he and his deputy, Lieutenant-General Raymond Odierno, were working to determine precisely how many troops would be required.

"The key is really how much force do you need? The campaign plan lays out the general concepts, the lines of operation and so forth and the actual plans and the actual force requirements are something that flow from that. And that's what General Odierno and I are working on now," Petraeus said.

Petraeus is due to report back to Washington in six weeks on the success of the "surge" -- an increase of U.S. troops Bush ordered to Iraq this year to help restore security, especially around the capital Baghdad.

He told ABC he expected to complete his assessment in time, after which he would be able to announce when troops can start to come home.

"We do think by about that time, again, that I will have enough of a sense ... to determine at what point we can in fact begin to send forces home without replacements," Petraeus said.