(Recasts lead, adds reaction and details)

By Richard Cowan and Thomas Ferraro

WASHINGTON, July 13 (Reuters) - Two of President George W. Bush's fellow Republicans in the Senate who seek a change of course in the war in Iraq urged him on Friday to draft plans to begin a possible troop withdrawal by the end of the year.

The proposal by Sens. Richard Lugar of Indiana and John Warner of Virginia, unlike troop withdrawal plans by leading Democrats, would leave it up to Bush to order any pullout.

While it has received a tepid response from Senate Democrats and the White House, the measure underscores the growing bipartisan opposition in the U.S. Congress to the increasingly unpopular war.

Lugar's Senate speech next week, a text of which was distributed to reporters, says the Bush administration should "immediately initiate planning for post-September contingencies, including a drawdown or redeployment of forces."

"We recommend that the president and the administration design plans to be executable beginning not later than Dec. 31," says the measure that will be offered as an amendment to a military policy bill now on the Senate floor.

Lugar and Warner are two of the most prominent Republicans in the Senate on foreign affairs and military matters. In recent weeks they have become increasingly critical of the war, leaving Bush with a more difficult path to continue combat, now in its fifth year.

Senate Republican Leader Mitch McConnell of Kentucky had no immediate comment, while the office of Senate Democratic Leader Harry Reid voiced caution.

"They (Warner and Lugar) clearly recognize there is no purely military solution in Iraq," said Reid spokesman Jim Manley. "But they put a lot of faith in the president - that he will voluntarily change course."

Manley added that Reid believes legislation is needed to force Bush to act.

WHITE HOUSE: GIVE IT TIME

White House spokesman Tony Fratto said the Lugar-Warner plan would be reviewed carefully.

"But we believe the new way forward strategy -- which became fully operational less than a month ago -- deserves the time to succeed," he said.

Fratto was referring to a U.S. troop buildup that began early this year to secure Baghdad from sectarian violence.

The two senators' initiative does not set deadlines for beginning the withdrawal of some of the nearly 160,000 U.S. troops now in Iraq or set a date for accomplishing a withdrawal, as Democrats want.

A showdown vote on a Democratic plan for removing combat troops from Iraq by next April is expected next week.

Under Warner-Lugar, Bush would present new war plans to Congress by Oct. 16 and those plans should be designed to be implemented no later than Dec. 31.

With their proposal, Lugar and Warner are joining several Democrats who have long argued that Congress' 2002 authorization of the Iraq war is obsolete and a new rationale for the war also should be submitted to lawmakers.

The senior Republicans call on Bush to submit a new argument for the war in September, when Gen. David Petraeus, the U.S. commander in Iraq, is scheduled to deliver a much-awaited report on progress toward securing Baghdad with recent boost of 30,000 more U.S. troops.

On Thursday, Warner said Iraq's government was "not providing leadership worthy of the considerable sacrifice of our forces and this has to change immediately."

A Lugar spokesman said he did not know whether the amendment would attract the 60 votes necessary in the 100-member Senate to advance controversial measures.

Lugar and Warner unveiled their amendment one day after Bush submitted an interim report to Congress on progress in Iraq that did little to lessen lawmakers' unease over the war.

It also came a day after a deeply divided U.S. House of Representatives voted to begin combat troop withdrawals within four months and complete them by April 1. The White House said Bush would veto such a bill.

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