Unlike the Republican platform, which has mostly been ignored outside of the abortion issue, the Paul Ryan budget is the core document of the 2012 campaign. It is the most explicit expression of the Republican agenda, endorsed by the party’s presidential candidate, Mitt Romney, and backed by decisive majorities of House and Senate Republicans.

That much is known. What people have not been talking about enough is that the Ryan budget contains an $897 billion sinkhole: massive but unexplained cuts in such discretionary domestic programs as education, food and drug inspection, workplace safety, environmental protection and law enforcement.

The scope of the cuts – stunning in their breadth — is hidden. To find the numbers, turn to page 16 of the Concurrent Resolution on the Budget – Fiscal Year 2013. In Table 2, Fiscal Year 2013 Budget Resolution Discretionary Spending, in the far right hand column, you’ll see the nearly $897 billion figure, which appears on the line marked “BA” for Budget Authority under Allowances (920) as $896,884 (because these figures are listed in millions of dollars).

According to the House Budget Committee, of which Ryan is the chairman:

The federal budget is divided into approximately 20 categories known as budget functions. These functions include all spending for a given topic, regardless of the federal agency that oversees the individual federal program. Both the president’s budget, submitted annually, and Congress’ budget resolution, passed annually, comprise these approximately 20 functions.

Within the 20 “budget functions” lurks — at number 19 — “Function 920.” In a masterpiece of bureaucratic obscurantism, the explanation provided by budget committee reads as follows:

FUNCTION 920: ALLOWANCES Function 920 represents a category called “allowances” that captures the budgetary effects of cross-cutting proposals or contingencies that impact multiple functions rather than one specific area of the budget. It also represents a place-holder category for any budgetary impacts that the Congressional Budget Office has yet to assign to a specific budget function. C.B.O. typically reassigns the budgetary effects of any legislation enacted within Function 920 once a new baseline update is released.

Mary Altaffer/Associated Press

The importance of the nearly $1 trillion in unexplained and unspecified cuts that Ryan and the Republican party are proposing, under the catch-all rubric of “Function 920: Allowances,” cannot be overestimated. These invisible cuts are crucial to the Republican claim that the Ryan budget proposal will drastically reduce the federal deficit (eliminating it entirely in the long run) and ultimately erase the national debt.

Ryan’s plan was passed 228-191 by the House on March 29, 2012, with no Democrats voting yes. On May 16, the Senate rejected the plan by a vote of 58-41. The vote among Senate Republicans was 41-4 in favor.

While the Ryan budget does specify cuts in programs serving the poor, many of whom are Democratic constituents (Medicaid, food stamps, unemployment benefits), it hides under the abstruse veil of “Function 920 allowances” the cuts in programs popular with many other voters.

This maneuver stands in stark contrast to Ryan’s campaign rhetoric. At a rally last Tuesday in Westlake, Ohio, Ryan declared:

We will not duck the tough issues. We will not kick the can down the road.

Romney and Ryan have made their willingness to stand tall and to confront forthrightly the problems facing the nation a central theme of their campaign. In Ryan’s words, again from Westlake:

We will lead. We will not blame others for four years; we will take responsibility and fix this country’s problems.

The lack of detail in the Ryan budget applies mainly to programs of importance to the voters Republicans continue to angle for, including swing voters concerned about programs like education, environmental protection and food safety.

Interviews I conducted with New Hampshire voters last month reveal the political liabilities of telling potential Republican voters exactly what the Romney-Ryan ticket intends to cut. Two voters, both Republicans, told me they could not bring themselves to vote for their party this year because the Ryan budget cuts spending for veterans’ benefits.

In an interview days after Romney announced on a Saturday that he had picked Ryan, George Lemieux said, “Based on what Romney did this weekend, I would not vote for him.” Lemieux, a 67-year-old Vietnam War veteran who spent 26 years in the Army, declared that “Ryan wants to decimate Medicare; he wants to decimate the V.A. I have a brother who is dependent on V.A. disability, and he wants to cut it out entirely.”

“The Ryan budget will kill everybody,” said Aura-Lee Nicodemus, another woman I met, who works at the V.A. Medical Center in White River Junction, Vt. and is active in the advocacy organization, Disabled American Veterans. “I’m a registered Republican and I can’t vote for Romney. His actions speak louder than words.”

There is a clear rationale for their concerns.

Under the Ryan budget, “Mandatory and Defense and Nondefense Discretionary Spending” – which includes Function 920 Allowances, but excludes Social Security, Medicare and Medicaid — would fall from 12.5 percent of Gross Domestic Product in 2011 to 6.75 percent in 2023, 5.75 percent in 2030, 4.75 percent in 2040 and 3.75 percent in 2050, according to an analysis by the Congressional Budget Office.

The C.B.O. cautiously notes how difficult it would be to cut such spending to 3.75 percent of G.D.P.:

By comparison, spending in this category has exceeded 8 percent of G.D.P. in every year since World War II. Spending for defense alone has not been lower than 3 percent of G.D.P. in any year during that period.

Romney, in fact, has committed himself to keeping the Pentagon budget (Function 050) at 4 percent of G.D.P. By 2050, that would leave zilch under the Ryan plan for such separately funded programs as Veterans Benefits (Function 700); the administration of justice, including the F.B.I. (Function 750); Education, Train and Social Services (Function 500), and pretty much anything else.

The big question posed by the comments of the two defecting Republicans I interviewed in New Hampshire is: are Romney and Ryan so committed to the principle of deficit reduction that they are willing, in an election year, to take on veterans? That would be extraordinary. The answer is no.

I emailed the Romney campaign. Here’s what I asked:

Talking to voters in New Hampshire, some veterans voiced strong concerns over the scope of likely cuts to the V.A. in the Ryan budget. Has Governor Romney said what will happen to veterans’ benefits under his administration?

The campaign immediately disputed any suggestion that the ticket supported cuts in services for veterans. Here is the Romney campaign’s emailed response:

That is false. Here are the facts: – The House-passed Fiscal Year 2013 budget matches the President’s discretionary request for veterans for fiscal year 2013: $61.3 billion. Over the ten-year window, the House-passed budget is actually above the President’s request on both the mandatory and discretionary side of the ledger. – On the mandatory side, the House Republican budget calls for $270 million more than President’s request. On the discretionary side, the House Republican budget calls for $16.4 billion more than President’s request, increasing America’s funding for services and benefits earned by veterans.

In an accompanying statement, Andrea Saul, the Romney campaign spokeswoman, said:

Gov. Romney opposes President Obama’s plan of drastic cuts to veterans’ benefits and the military while exploding the federal budget elsewhere. President Obama’s own V.A. Secretary has admitted that Obama’s devastating defense budget cuts put veterans’ funding at risk for an arbitrary across-the-board cut. Gov. Romney and Paul Ryan are committed to keeping faith with our veterans and providing the care they so richly deserve.

Hmm. How does this fit with the deficit-reducing claims of the Ryan budget and with Ryan’s boast that “We will not duck the tough issues? We will not kick the can down the road?”

It turns out that a reading of the Ryan budget — if you don’t parse Function 920 — is deceptive. In the case of veterans’ benefits, for example, Andrea Saul’s claim that the Romney-Ryan ticket is “committed to keeping faith with our veterans” appears, on the surface, to be legitimate, because none of the mysterious Function 920 cuts show up in her computations.

If veterans’ benefits are to be protected, what programs will be on the chopping block to achieve the $897 billion in cuts listed under the mysterious “Function 920 allowances” category? Will it be education or food inspectors, air traffic controllers or homeland security?

The Ryan budget does, in fact, “duck the tough issues.” Ryan claims to be proposing major steps toward a balanced budget and long-term debt reduction, but he doesn’t really tell voters how he is going to get there.

Interestingly, the budget proposed by President Obama does specify where cuts would be made, including those called for in the Budget Control Act, the measure approved by Congress and signed into law on August 2, 2011, as part of the deal to raise the debt ceiling and to avoid default on government debt.

A statement in the Obama administration budget claims credit for making explicit the “difficult trade offs” to reach spending reduction goals:

In the Budget Control Act, both parties in Congress and the President agreed to tight spending caps that reduce discretionary spending by $1 trillion over 10 years. This budget reflects that decision. Thus, for all the priority areas we are investing in, difficult trade-offs had to be made to meet these very tight caps. Discretionary spending is reduced from 8.7 percent of G.D.P. in 2011 to 5.0 percent in 2022.

Perhaps the most striking aspect of the omissions in the Ryan budget is the failure of Obama and other Democrats to capitalize on it.

Leading Democrats I spoke to, who refused to be identified because they did not want to be quoted faulting their own party, cited two factors limiting their ability to mount a counter-attack. First, the complexity of the issue makes it difficult for reporters to understand and write about the subject. After wading my way through all of this, I know what they mean. Second, the Ryan tactic of obscuring the cuts successfully plays to a fundamental ambivalence that amounts to an internal contradiction in public opinion: strong support for spending cuts in the abstract, but opposition to many specific cuts in programs that have popular support.

In a speech on April 3 at the Associated Press luncheon in Washington, Obama tried to make the case against Ryan well before he was picked to run for vice president. Applying the $897 billion in cuts under “Function 920 Allowances” to domestic spending programs, Obama projected a future scenario:

The year after next, nearly 10 million college students would see their financial aid cut by an average of more than $1,000 each. There would be 1,600 fewer medical grants, research grants for things like Alzheimer’s and cancer and AIDS. There would be 4,000 fewer scientific research grants, eliminating support for 48,000 researchers, students, and teachers. Investments in clean energy technologies that are helping us reduce our dependence on foreign oil would be cut by nearly a fifth. If this budget becomes law and the cuts were applied evenly, starting in 2014, over 200,000 children would lose their chance to get an early education in the Head Start program. Two million mothers and young children would be cut from a program that gives them access to healthy food. There would be 4,500 fewer federal grants at the Department of Justice and the F.B.I. to combat violent crime, financial crime, and help secure our borders. Hundreds of national parks would be forced to close for part or all of the year. We wouldn’t have the capacity to enforce the laws that protect the air we breathe, the water we drink, or the food that we eat. Cuts to the F.A.A. would likely result in more flight cancellations, delays, and the complete elimination of air traffic control services in parts of the country. Over time, our weather forecasts would become less accurate because we wouldn’t be able to afford to launch new satellites. And that means governors and mayors would have to wait longer to order evacuations in the event of a hurricane.

Ryan, in the meantime, remains consistent. Two days after his speech in Westlake, on Sept. 6, he reiterated his claims at a rally in Colorado Springs:

So here is our commitment. We are not going to duck the tough issues and kick the can down the road. We are going to lead and fix this mess in Washington. And we are not going to spend the next four years blaming people from the last four years. We’re going to take responsibility and get the job done, reach across the aisle and fix this problem, get people back to work, create jobs, growth.

In an interview, Christopher Van Hollen Jr. of Maryland, the ranking Democrat on the House Budget Committee, told me that the Ryan budget “is a shell game designed to hide the damage to the country.” Van Hollen is frustrated that the damage to which he alludes has not become a campaign issue: “The magnitude of this budget gimmick takes your breath away.”

Thomas B. Edsall, a professor of journalism at Columbia University, is the author of the book “The Age of Austerity: How Scarcity Will Remake American Politics,” which was published earlier this year.