Two powerful congressional committees will start their work crafting the Pentagon’s budget next week.

The Senate Appropriations Committee (SAC)–the only of the four panels that writes the defense budget that has not done so yet for fiscal year 2012–said yesterday it will meet Sept. 7 to set the so-called 302(b) allocations for the 12 federal appropriations bills, including defense.

Also next week, the 12 members of the new congressional Joint Select Committee on Deficit Reduction, charged with identifying up to $1.5 trillion in long-term government savings this fall, will be together in Washington for the first time since their panel was created via the Budget Control Act of 2011, which President Barack Obama signed Aug. 2. The deficit committee is expected to schedule its first hearing, which could be next week and under the law must be by Sept. 16.

The SAC had been waiting for more clarity from the White House on deficit-cutting talks before setting its 302(b) allocations. Other lawmakers had anticipated that the already-crafted FY ’12 defense proposals on Capitol Hill would need to be revised to reflect the deficit deal Congress and Obama ultimately reached last month. FY ’12 begins Oct. 1.

The Budget Control Act of 2011 calls for slashing $350 billion in defense-related spending over the next decade, according to the White House’s interpretation. It also charges the new 12-member congressional committee finding up to $1.5 trillion in additional government savings. If it and Congress can’t agree on a plan by the end of the year, $1.2 trillion in overall spending automatically will be cut, with half of that coming from a category of defense spending defined as mainly the Pentagon budget.

The law does not specify a figure for the FY ’12 Pentagon budget as part of the first wave of $350 billion in defense-related cuts. Yet the act does state that FY ‘12 “security” funding–including the Pentagon and the departments of homeland security, state, and veterans affairs, along with other related spending–is capped at $684 billion.

Back in February, the White House requested a $553 billion base Pentagon budget for FY ’12. Some pundits have estimated that figure could be revised to roughly $525 billion because of the new deficit-cutting law.

The House already passed a FY ’12 defense appropriations bill that the House Appropriations Committee (HAC) said cut $9 billion from the official Pentagon request. A House-passed defense authorization bill, though, called for no cut to the base defense budget. Meanwhile, the Senate Armed Services Committee already approved its version of the FY ’12 defense authorization bill, which it said cut $5.9 billion from the Pentagon proposal, though Chairman Carl Levin (D-Mich.) has said the amount of his committee’s bill will have to be revised.

Lawmakers who have been in recess since the deficit law was signed have been seeking more clarity on its impact on the defense budget.

On Aug. 10, House Armed Services Committee (HASC) Chairman Howard “Buck” McKeon (R-Calif.), HAC Defense subcommittee (HAC-D) Chairman C.W. “Bill” Young (R-Fla.), and House Budget Committee Chairman Paul Ryan (R-Wis.) asked for more information on the defense budget in a letter to White House Office of Management and Budget Director Jack Lew and Defense Secretary Leon Panetta.

They requested information including an update on the status of any preliminary conclusions from a comprehensive Pentagon roles-and-missions review, which has been underway in recent months, so they can discern what could impact the defense budget as they work on the FY ’12 defense bills.

Meanwhile, the Joint Select Committee on Deficit Reduction’s six Democrats, led by Sen. Patty Murray (D-Wash.), talked over the phone for the first time Wednesday, a day after the six Republicans, led by Rep. Jeb Hensarling (R-Texas), had an initial meeting. The committee also announced Tuesday that Mark Prater will serve as its staff director. He has been serving as deputy staff director and chief tax counsel on the minority staff of the Senate Finance Committee.