President Obama has threatened to veto any appropriations bill adopted by Congress that would cap agency spending at lower levels than agreed to in the bipartisan deal to raise the nation’s debt limit last summer.

The threat raises the possibility of another Congressional clash over spending that could trigger another government shutdown as the fiscal year winds down — this time on the eve of the November election.

Republican Rep. Paul Ryan’s budget calls for stricter spending caps than agreed upon in last summer’s debt deal. (AP PHOTO)

“These funding levels will mean deep and painful cuts in investments America needs to succeed — in education and training, in research and development, and in clean energy and infrastructure-- and will undermine future economic growth and degrade many basic government services on which the American people rely,” Acting Office of Management and Budget Director Jeffrey D. Zients wrote to House Appropriations Chairman Hal Rogers (R-Ky.).

Zients said the House budget “breaks our bipartisan agreement.”

“Until the House of Representatives indicates that it will abide by last summer’s agreement, the President will not be able to sign any appropriations bills,” Zients concluded.

The veto message came as the House Appropriations Committee prepares to consider the first two of 12 appropriations bills that outline spending for individual federal agencies. The measures are geared to the overall spending level set in the Ryan budget.

Ultimately, the spending level issue will have to be worked out between appropriators in the GOP-led House and the Democratic-led Senate. The House bills will not emerge from Congress without changes, and the president will not have to choose whether to sign them unchanged.

But the veto threat raises political pressure on a process that was destined to be sensitive.

Sen. Kent Conrad (D-S.D.), chairman of the Senate Budget Committee. (William B. Plowman, NBC/AP)

“This year, when Appropriations bills pass both the House and the Senate, the President can choose to sign them, or else he can choose to shut down the federal government, put our people at risk, and imperil our economic recovery,” said spokeswoman Jennifer Hing.

The letter from the White House came as the Senate’s Budget Committee devoted a session Wednesday to considering a budget blueprint drawn up the day before by its Democratic chairman, Sen. Kent Conrad (S.D.).

Conrad, overseeing his final budgeting year before retiring from the Senate next year, convened the meeting to discuss a 10-year budget plan modeled on the suggestions of a 2010 presidential fiscal task force that would reduce deficits by more than $4 trillion over the decade through a combination of spending cuts and new taxes. But bowing to the political realities of an election year and the wishes of Democratic leadership, Conrad did not allow the committee’s Republicans to offer amendments or ask that the committee vote on his suggestion or send recommendations to the full Senate.

Democrats and Republicans instead spent the meeting mired in debate over Congress’s gridlocked budgeting process.

The decision by Democrats not to pursue a full, 10-year spending plan means the Senate will fail to adopt such a plan for a third year a row -- a record since Congress adopted budget process reforms in 1974. Republicans have made this failure a centerpiece of their efforts to retake the Senate in november, and they took turns bashing it during the non-voting session.

Before the meeting opened, Sen. John Thune (R-S.D.) told reporters it would be “a dog and pony show--without the dog and pony.”

Sen. Ron Johnson (R-Wis.) termed it “an unfortunate charade.”

“An absolute failure of leadership,” said Sen. Kelly Ayotte (R-N.H.). ”If you can’t pass a budget, you should be fired,” she said.

Democrats insisted that the debt deal wrote spending levels into law, meaning there is no need to adopt a new budget resolution for this year. It made more sense, the said, to discuss how to get a bipartisan agreement on a long-term plan to curb the nation’s spending trajectory.

After the election, Congress will face two Jan. 1 deadlines that could compel action--the expiration of the Bush-era tax cuts and the triggering of the deep automatic spending cuts agreed to in the debt deal.

“It is time to move off our fixed positions,” Conrad said. “I understand that’s probably unlikely right before an election. But before the end of the year, we’re going to have to find a way to come together.”

Staff writer Lori Montgomery contributed to this report.