Random speculation: Is part of Reid's gambit in withdrawing the defense bill and limiting amendments part of a game of senatorial chicken? Reid's been pretty careful about keeping half-way amendments (amendments urging a change in policy for the administration without requiring any withdrawal) from getting on the floor during this debate--at least as long as the Levin-Reed forced troop withdrawal was on the table. After all, as this Washington Post story observes:

Matzzie said his group's [Moveon.org] efforts are concentrated on "forcing the entire Republican Party to look over the side of the cliff" at the political consequences of continuing to stand by Bush. Antiwar groups are focused in particular on Senate Republicans up for reelection next year.

"Ultimately, we end the war by creating a toxic political environment for war supporters like the Republicans in the Senate," Matzzie said.

So could one attempt to create this "toxic environment" be denying uncertain Republicans the use of half-way measures, which would allow them to vote to "change course" without forcing withdrawal? By only allowing them to vote on Levin-Reed-style withdrawal amendments, Reid can attempt, if they continue to vote against such withdrawals, to bash them over the head with the claim that they continue to support only "stay the course." Of course, that tactic isn't completely consensus-building, but how much does Reid care about "bipartisan consensus" and how much does he care about forcing withdrawal and scoring political points to force this withdrawal?
Of course, this game of chicken could backfire politically--especially if the public begins to believe that the president's new Iraq policy is working. It could also backfire tactically for Reid (assuming he really does value a withdrawal over seeming temporary political advantage): his dualistic approach (withdrawal v. stay the course) could stiffen certain undecided Republicans' necks and prod them to stand with the president still--while a more consensus-based approach could slowly guide undecided Republicans in the direction of supporting forced withdrawal. This approach could also be a very bad idea in practice for the US and its foreign policy aims, and maybe forced withdrawal could be very bad policy in terms of facts on the ground in Iraq. But we're covering Capitol Hill politics here--let's not get too concerned with facts on the ground (who knows how much they figure into various political calculations)!
Of course, there's another side to this cliff, too, and the Republicans are beginning to emphasize that. By denying a vote on the Defense bill, Reid is also denying a vote on, as Sen. Cornyn puts it,

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A 3.5 percent across-the-board pay raise for all uniformed service personnel effective January 1, 2008.

Authorization to provide $4 billion for Mine Resistant Ambush Protected, or MRAP, vehicles to protect our service members from Improvised Explosive Devices (IEDs) in Iraq and Afghanistan.

Authorization to provide over 25 critically important bonuses and special pays set to expire at the end of this calendar year (e.g., enlistment and reenlistment bonuses and health care professional incentive pays).

The Dignified Treatment of Wounded Warriors Act, which addresses support programs for wounded combat veterans and their families in a comprehensive manner, including numerous initiatives to address traumatic brain injury (TBI) and post-traumatic stress disorder (PTSD) specifically. This was unanimously adopted by amendment to the bill.

Authorization of new hiring and bonus authorities to assist the Department in recruiting and retaining needed health and mental care professionals.

Authorization to add $2.7 billion for items on the Army Chief of Staff’s Unfunded Requirements List, including

• $775.1 million for reactive armor and other Stryker requirements,
• $207.4 million for aviation survivability equipment,
• $102.4 million for combat training centers, and
• Funding for explosive ordnance equipment, night vision devices, and machine guns.

Authorization of $50 million in supplemental educational aid to local school districts affected by the assignment or location of military families

Authorization of payment of combat-related special compensation to service members who are medically retired due to a combat-related disability.

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So we almost have two games of chicken here. And who knows who's going to win!
(Of course, all this speculation could become meaningless at any moment--depending on when/if/how Reid plans to bring this bill back...)


Update 6/20: WaPo has some more comments on this game of chicken:

Moving to other Iraq proposals, Schumer said, while Republicans were blocking the Democrats' first choice, would have provided Republicans with a safe haven from taking the tough stance that is required to end the war.

"It would delay them coming on board, because they would say [to their voters], 'See, I'm trying to do something,' " Schumer said.

And:

Yet, at the same time, Democratic war opponents are agitating for party leaders to push aggressively to bring troops home. Sen. Richard J. Durbin (Ill.) explained the leadership's rationale this week. "Was it worth it? I think it was. Now the Senate's on record. Many senators who've gone home and said they're opposed to the war voted to continue the war. They'll have to answer to the voters."

But Sen. Biden might not like the chicken game as much:

The Salazar-Alexander plan is an example of the sort of bridge measure that could lure Republicans to break from Bush -- a process some Democrats have noted is probably going to be a gradual process. "They need something to jump onto first," said Sen. Joseph R. Biden Jr. (D-Del.), chairman of the Senate Foreign Relations Committee and a presidential candidate.

Meanwhile, Republicans and some Democrats are pushing back and complaining about the "rudeness" of Reid's tactics--and hinting that, as Sen. McCain put it,

By not passing the defense authorization bill, we are abandoning our men and women in the military. By not passing this legislation we are not allowing a pay raise; modernizing our soldiers’ military equipment; nor are we passing the wounded warriors legislation, which we all know is vitally needed to care for our wounded veterans.

Meanwhile, the NYT describes the current Senate condition as a "food fight" (without the food).