When it comes to curbing the massive federal deficit, there’s no problem knowing what needs to be done.

It’s figuring out how — and when — to do it.

Everyone knowledgeable understands it requires cutting future costs of so-called “entitlements” like Medicare, Medicaid and Social Security, which many Democrats viscerally oppose doing. It means providing additional revenue and curbing military spending, which Republicans viscerally oppose doing.

This is the real bottom line behind the congressional battle over cutting the current budget or even some of the talk about next year’s spending plan. Though billions are involved, and lawmakers ultimately must provide federal funding, the fight is largely political and enables everyone from President Barack Obama to the greenest House freshman to show voters how serious he or she is about cutting spending.

While it may have significant political fallout, it won’t do much to curb the long-term deficit because they’re fighting over 15 percent of the budget, which funds discretionary domestic programs.

Despite the fiery rhetoric, the amount involved is relatively small in a budget that has reached $3.7 trillion, with a $1.6 trillion deficit. Even if all House cuts were adopted, and top Republicans concede they won’t be, the total is $61 billion or, on an annual basis, $100 billion.

Though some amendments cut well-meaning but non-essential programs, like $1.5 million for “greening” the Capitol, others would do real damage by trimming funds for college loans, monitoring clean air and enhancing border enforcement.

The impasse may lead to another of those theatrical government shutdowns like 1990, preceding enactment of a deficit reduction package under the first President Bush, or the 1995 one forced by another crop of rambunctious House GOP freshmen.

But assuming far-ranging policy riders like the six barring funds to implement Obama’s health care program are ultimately dropped, this is really a prelude to the real battle over reining in the long-term deficit.

And it’s not yet clear when that will unfold.

After a majority of Obama’s bipartisan deficit control commission in December backed its sweeping plan, many of us hoped the president would seize the political high ground by including it in his 2011 agenda.

He didn’t, drawing fire from pundits and bipartisan critics accusing him of forfeiting leadership. But he made clear he believes anything he proposed would immediately become a target for Democrats opposed to curbing most entitlement programs and Republicans opposed to most anything he might favor. His preferred course: Let partisan battles play out and help create a climate for bipartisan action.

In recent weeks, quiet bipartisan talks have begun in the Senate, perhaps the likeliest place for progress since it is inherently less polarized. Besides, a majority of the debt panel’s Democratic and Republican senators endorsed its proposals.

One idea is to use the panel’s plan to force necessary cuts.

Predictably, these efforts immediately encountered problems. Top Senate Democrats are trying to bar any Social Security reductions. And Obama’s failure to propose any long-term plan is spurring House Republicans to propose one in their 2012 budget that may well go beyond anything that can be enacted.

Actually, Social Security may be the easiest part. Top leaders of both parties have already suggested raising the retirement age gradually to 70 and limiting future benefits for wealthier recipients.

It will be harder to curb Medicare, Medicaid and defense spending and to increase revenues, perhaps through a tax simplification plan that limits deductions for wealthier taxpayers. Polls show the public opposes Social Security and Medicare cuts.

Two key senators, Tom Coburn, R-Okla., and Claire McCaskill, D-Mo., agreed on “Fox News Sunday” that “everything” has to be part of the package.

But a lot of political sound and fury, and possibly another election, will have to come first.