WASHINGTON (CNN) - Republicans in Congress aren't the only lawmakers President Barack Obama will have to win over as he pitches his budget on Capitol Hill.
In a letter to the Senate Budget Committee dated Tuesday, 12 of the 16 members of the centrist Senate Democratic coalition - which calls itself "the moderate Dems Working Group" - expressed grave concerns about the direction of the president's $3.6 trillion budget.
The lawmakers note the Congressional Budget Office's projection of a cumulative deficit over the next 10 years of $9.3 trillion, "roughly $2.3 trillion higher than the president's budget had assumed."
The letter was addressed to Budget Committee Chairman Sen. Kent Conrad, D-North Dakota, and ranking Senate Budget Committee member Sen. Judd Gregg, R-New Hampshire. Both the Senate and House Budget Committees begin the work of drafting a budget blueprint Wednesday.
While acknowledging the economic and fiscal distress that the Obama administration has inherited, the 12 moderates say the size of the deficit projections "are not acceptable."
"When the administration examines the new data, we believe they will agree that modifications must be made to the president's budget that are both consistent with his principles and reflective of the ever-worsening fiscal reality."
The authors of the letter also noted that the nation's Social Security program must be reformed, along with the Medicare and Medicaid programs; the tax system is due for an overhaul and spending should be restricted to the goals of job creation and economic recovery.
The moderate Democrats are becoming a force to be reckoned with. One of them, Sen. Mark Pryor, D-Arkansas, told CNN, "There is a lot of diversity in the Democratic party. None of us were elected by Barack Obama. We don't represent the president, we represent our states."
The group's concerns virtually assure that the president's spending agenda will fall substantially short of filibuster-proof support in the Senate, which is why Obama traveled to Capitol Hill to state his case for his budget to Senate Democrats.
Analyst Norm Ornstein, of the American Enterprise Institute told CNN, "There are large numbers who are moderate, centrists, even a little bit conservative who are not going to let some sweeping, very liberal policy go through the Senate. There aren't sixty. And in fact, moderate Democrats provide
the kind of bridge required to get a significant number of Republicans over to make some of these major areas of policy."
The senators who signed the letter were: Sen. Tom Carper, D-Delaware; Sen. Evan Bayh, D-Indiana; Sen. Blanche Lincoln, D-Arkansas; Sen. Mark Pryor, D-Arkansas; Sen. Kay Hagan, D-North Carolina; Sen. Jeanne Shaheen, D-New Hampshire; Sen. Herb Kohl, D-Wisconsin; Sen. Mary Landrieu, D-Louisiana; Sen. Claire McCaskill, D-Missouri; Sen. Bill Nelson, D-Florida and Sen. Mark Begich, D-Alaska. Sen. Joe Lieberman, formerly a Democrat and now an Independent from Connecticut, also signed the letter.