Nebraska’s two maverick Senators — one a Democrat, the other a Republican — tipped the scale in favor of Democrats on Tuesday in a crucial reversal of the chamber’s showdown over whether to force President Bush to curtail U.S. involvement in Iraq.

Switching their votes to support Democratic language setting a nonbinding goal for the United States to end all combat missions in Iraq by March 31, 2008, Sens. Chuck Hagel (R) and Ben Nelson (D) provided Majority Leader Harry Reid (D-Nev.) with the two votes he needed to keep the withdrawal language in a $121.6 billion supplemental war spending bill. The vote came just hours after Bush threatened to veto the measure.

Two weeks ago, the duo’s votes instead helped Republicans deliver a 48-50 defeat to Reid’s attempts to limit the U.S. mission in Iraq.

This time the 50-48 vote came out in Reid’s favor as the Senate rejected a Sen. Thad Cochran (R-Miss.) amendment that would have stripped the timetable from the bill, along with a requirement that Bush begin redeploying troops within months of enactment.

After the vote, Reid praised Nelson and Hagel for their switch. “I so admire the courage of the new voters we had on this,” he said.

Nelson’s turnabout was perhaps the most surprising, given that Hagel, a fierce critic of the White House’s handling of the Iraq War, indicated Sunday that he likely would not continue to support the Senate GOP leadership position that setting a target date for a troop pullout would be like telling insurgents when the U.S. will surrender.

Though Nelson joined all but one Republican March 15 in rejecting the entirety of the Democratic Iraq resolution, he said he still was struggling a few hours before Tuesday’s vote on what to do. Then, just a half-hour beforehand, Nelson announced in a press release that he would oppose the Cochran amendment, despite his opposition to setting a “calendar date for withdrawal from Iraq.”

“While the intent may be to remove the nonbinding withdrawal date, the amendment goes beyond that,” Nelson said in the statement. “It would also remove several other critical provisions, including language acknowledging that the situation in Iraq has become a civil war and calling for diplomatic and political engagement on the part of both the Iraq and American governments.”

Ending speculation about his plans, Hagel took to the floor Tuesday and declared that the Iraq resolution in the war spending bill was the “responsible way” for Congress to assert its prerogatives.

“This idea that somehow you don’t support the troops if you don’t continue in a lemming-like way to accept whatever this administration’s policy is, that’s what’s wrong,” Hagel said. “This language establishes a limited U.S. military mission in Iraq that focuses on the things that we should be doing.”

Prior to the vote, Democrats reached out to both Democrats and Republicans who voted against the resolution previously, said Majority Whip Dick Durbin (D-Ill.).

Durbin said rehashing the issue was “an important vote for us because it will give us a good signal of whether there will be meaningful Iraq language in this bill.” Indeed, language limiting the U.S. mission in Iraq almost certainly will be sent to the president after the two chambers reconcile their different versions.

But Durbin indicated that the House’s language setting an August 2008 date certain for withdrawal from Iraq was unlikely to pass muster in the Senate. “We’re going to have to work out a compromise in conference,” he said.

Even as the Democrats celebrated their victory, the specter of a presidential veto hung over the gaiety.

The White House formally sent up its Statement of Administration Policy on Tuesday, vowing a veto because, “This bill assumes and forces the failure of the new strategy even before American commanders in the field are able to fully implement their plans. Regardless of the success our troops are achieving in the field, this bill would require their withdrawal. This and other provisions would place freedom and democracy in Iraq at grave risk, embolden our enemies, and undercut the Administration’s plan to develop the Iraqi economy.”

But Senate Armed Services Chairman Carl Levin (D-Mich.) said that for all the president’s objections to setting a timetable for withdrawal, Bush actually has told him personally that it is “useful” for the Iraqis to hear that support in the U.S. is waning for a continuing troop presence in Iraq.

“What we’re doing here is giving [the White House] leverage with the Iraqi leaders,” Levin said, indicating that the president is using the Democratic “bad cops” position to pressure Iraqis to improve training and readiness of their own troops and security personnel.

Levin said that, despite the president’s private acknowledgements that the Democrats’ Iraq language is serving a constructive purpose, “he’ll veto it, because he wants to be the good cop.”

Republicans tried to put a brave face on their loss Tuesday.

“What I think the Democrats have done is overplayed their hand on this, because they’re the ones who look irrational,” said Sen. John Thune (R-S.D.). Thune said the GOP’s message that the Democrats’ war spending bills would announce the United States’ surrender date to its enemies is working “surprisingly well.”

One Senate GOP leadership aide said the outcome of the vote was not as important as the positive reaction Republicans believe they will get for having held their ground.

“We are going to end this week … having stood against pork spending and the micromanaging of our commanders in Iraq,” the aide said. “And at the end of the day, the president still vetoes it, and we still win.”

Senate Minority Leader Mitch McConnell (R-Ky.) explained that Senate Republicans ultimately would acquiesce to including a timetable, suggested or required, for withdrawal from Iraq because, “The president is in a very strong position on this.”

And one Bush administration official said the veto may have made Nelson and Hagel believe that they had “freebie votes.”

“It’s a strategic risk to have a veto threat out there so early on,” said the official of Bush’s unofficial veto threat weeks ago. “People throw away their votes because they know it’s going to be stripped out anyway.”

Despite having dispensed with the Iraq resolution, Republicans said they would continue to rail against the bill and its nearly $20 billion in domestic spending.

McConnell pointed out that the Iraq language was not the only provision that drew a veto threat from the president. Bush’s Statement of Administration Policy states that, “Because of the excessive and extraneous non-emergency spending it contains, if this legislation were presented to the President, he would veto the bill.”

Sen. John McCain (R-Ariz.) ridiculed the bill’s inclusion of $2.5 million for Capitol Hill visitor tours. “Apparently our friends are more interested in tours of the Capitol than in military tours in Iraq,” he said.

Erin P. Billings contributed to this report.