The U.S. government must shrink if it wants to return to a fiscally sustainable course, Sen. Kent Conrad (D-N.D.) said Wednesday.

Conrad, the chairman of the Senate Budget Committee and a member of President Obama's fiscal commission, said that there was no other choice but to cut spending in order to balance the budget.

"This government is going to have to be smaller," Conrad said on MSNBC. "There is no option."

Conrad, the top Democrat on budgetary issues, is among the 18-member fiscal commission Obama appointed earlier this year to generate recommendations on how to bring U.S. deficits and debt under control.

The commission, which is set to announce its recommendations in December, has received a pledge from House and Senate leaders to allow a vote on its prescriptions.

But lawmakers on both sides of the aisle have begun to box in the group over what it might announce.

Liberal Democrats — including more than half of the members of the House — wrote Obama about the commission on Tuesday, threatening to vote it down if the commission recommends significant changes to Social Security. Republicans, meanwhile, have warned against new taxes that might emerge from the commission.

Conrad said that "everything does have to be on the table," in terms of options available to the fiscal commission, but that the bulk of cuts would have to be targeted at military and entitlement spending.