The Senate was poised to vote Thursday on a massive bill setting defense policy for fiscal 2010, after adopting an amendment aimed at holding off a veto of the measure.

“We want to do our best to finish it today,” Majority Whip Richard J. Durbin , D-Ill., said, suggesting that debate could continue into the weekend if Republicans don’t cooperate.

“I hope that doesn’t happen. It doesn’t have to happen. But we want to get this done,” he said.

Senators adopted by voice vote an amendment by Joseph I. Lieberman , I-Conn., to bar continued spending on an alternative engine for the F-35 Joint Strike Fighter until the Defense secretary certifies that such a program would reduce the fighter program’s costs, improve the planes’ readiness and not disrupt the program’s development or result in fewer fighters procured.

President Obama had threatened to veto the bill over the provision authorizing $439 million for the backup engine if he believes it would “seriously disrupt” the overall program.

The F-35’s first engine is made by Pratt & Whitney largely in Connecticut and the alternate is being developed by General Electric and Rolls Royce in Ohio, Indiana and elsewhere. Congress has appropriated $2.5 billion on the engine program, said Senate Armed Services Committee Chairman Carl Levin , D-Mich.

Lieberman argued that the alternative engine would suck up money that should be used to buy more of the warplanes, citing a letter Wednesday from Defense Secretary Robert M. Gates .

The Senate rejected 38-59 a competing amendment by Evan Bayh , D-Ind., to withhold 10 percent of the funding for the F-35 unless sufficient money has been made available to continue development of an alternate engine. Bayh’s amendment would pay for the engine by decreasing by $439 million the authorization for C-130 transport planes for special operations forces in Iraq and Afghanistan. Bayh said the planes were already funded by the latest war supplemental spending measure.

But John McCain , R-Ariz., backed Lieberman, by countering that the competition already occurred and the Pratt and Whitney engine won.

“We’re actually talking about getting another bite at the apple,” McCain said.