A Senate panel approved a $575.9 billion defense spending bill on Tuesday, but its fate is uncertain as lawmakers argue over spending caps. 

The usually bipartisan bill has run into snags this year. The GOP-led House and Senate both have bills that give President Obama the amount he seeks for defense, but they do that by padding a war-fighting account that's not subject to the automatic spending caps that took effect a few budget cycles ago. 

Democrats and the White House -- and some Republicans -- say doing an end-run around the caps by increasing the war-fighting account for a year doesn't permit defense officials the flexibility to plan to keep U.S. military might strong, especially at a time when Islamic extremists are on the rise. It takes years, for instance, to develop weapon systems. 

The defense subcommittee of the Senate Appropriations Committee on Tuesday recommended $489.1 billion in base spending and another $86.8 billion for the war-fighting account called Overseas Contingency Operations. The total $575.9 billion measure is to be considered by the full committee on Thursday. 

If additional mandatory spending and money used in national security programs at the Energy Department are added in, the subcommittee's markup is aligned with the $612 billion defense authorization bills. 

Illinois Sen. Dick Durbin, the top Democrat on the subcommittee, said this year's defense appropriations process is overshadowed by a "budget impasse." 

Senate Democrats have said they will block the defense appropriations bill from the Senate floor unless there is agreement on spending caps. Obama has threatened a veto, saying he's opposed to using a budget "gimmick" to increase defense spending while failing to do away with the Budget Control Act's spending caps. 

"The Budget Control Act, which set in motion dangerous defense cuts, establishes caps on defense and nondefense discretionary spending," Sen. John McCain, R-Arizona, the chairman of the Senate Armed Services Committee, wrote in an op-ed published on Tuesday. "There is bipartisan consensus on the dangerous impact these spending caps would have on defense. All of the military service chiefs testified this year that funding defense at the level of the BCA caps would put American lives at risk. 

"Rather than seeking to avoid this scenario at all costs, the president is using it as leverage to extract increases in nondefense spending. As his veto threat made clear, the president `will not fix defense without fixing non-defense spending." 

The lengthy debate over the authorization bill has stretched into its second week on the Senate floor. The Republican-led Senate rejected an amendment introduced by Sen. Jack Reed, D-R.I., that would have put restrictions on the amount of funding available for the war-fighting account unless money for domestic programs received equal increases. 

The party-line vote was 51-46. 

Sen. Chuck Schumer, D-New York, said the rejection of the motion was a "harbinger of bad things to come." 

He said Democrats would allow the defense policy bill to move forward, but they have decided to block movement of any appropriations bills until a budget agreement is made. "Sooner or later, the Republicans are going to have to back off," he said.