For years the hackneyed line about Social Security was that the subject was the third rail of American politics: Touch it and you die an utterly horrible, utterly senseless death. No one voluntarily took on this issue.

Then again, no president, with the possible exceptions of James K. Polk and William McKinley, made pre-emptive strikes part of his diplomatic repertoire either.

So into the breach we go, solving problems that reach into another decade even as our own deficit (fueled by that discretionary war) reached $412 billion in the last fiscal year and, according to a summer 2004 Congressional Budget Office forecast that assumed federal spending will grow with the economy, will be basically the same figure ($414 billion) in fiscal 2009.

The economists can debate whether taking on this issue is prudent right now, with other financial issues — including the futures of Medicare and Medicaid and growing rebellion over the broadening reach of the Alternative Minimum Tax — beckoning. The political scientists can only salivate at the notion of exploring the philosophy behind Social Security, for it raises every important contested principle of government known to humankind.

Like, to take just one: Personal freedom vs. collective responsibility.

Whole books have been written on that subject, whole college courses have been dedicated to examining that topic, whole lives have been obsessed with resolving the difficult questions that grow out of it. And the president wants the Congress, notoriously reluctant to grapple with the hard issues, to wrestle with this issue, without the pressure of imminent crisis, and to reach a decision in about 20 months. Pretty neat trick, if you can do it. It’s never been done before.

Recognizing that some thinkers’ entire life’s work has just been brushed off in three sentences and a sentence fragment, let’s move on to the more crass aspects of this — not the ultimate Ways and Means measurement, taken by lobbyists with yellow legal pads and wallets padded by special interests, of who wins and who loses financially, but the political measurement, which in a far different way can be distilled down to who wins and who loses at the ballot box.

Make no mistake: The new battle over Social Security is a struggle for political power. Republicans know this, for that is why so many of them sought political assurances from the president when they met at their Greenbrier retreat last month. Fiddling with Social Security in the past has meant fiddling with GOP House members’ own security, and though few of them were in the Capitol in 1982, when the Democrats flourished at the polls by vowing to preserve Social Security, none of them is unaware of the lessons of that midterm election.

But the new Social Security issue is also a struggle for the future of American politics. The Democrats ran in 1982 as the protectors of Social Security because the demographics told them they should. Voters over 60 were the Democrats’ second-best age category in the 1980 election, and just as hunters shoot where the ducks are, politicians hunt where their votes (already) are. Add to that the pattern, established even then, that older Americans are more likely to vote than younger Americans, and the Social Security strategy made perfect sense.

The party that is voluntarily giving Social Security a central role in the national conversation won the support of voters 60 or over by 8 percentage points, its largest margin of victory in November’s election. That tells us two things: The first is that the Republicans feel a sense of confidence about older voters and the GOP ethos. (Who knows what assurances lurk in the internals of Republican polling figures?) The second is that the president’s plan almost certainly won’t substantially affect current Social Security beneficiaries.

Now let’s consider the Democrats’ perspective in this matter. They lost older voters in 2004, so they are hoping that they might squeeze some votes out of adopting their usual protective profile on Social Security.

But if their eye is on the prize, then they are looking at the biggest bulge in the population, the (aging, and none too gracefully) baby boomers, now roughly between the ages of 40 and 58. In the last presidential election, the Republicans barely captured the votes of this group, the margin closely approximating the nationwide 51-48 percent split.

So the Democrats will lay down their markers where it will do them the most good, and where they most need to do well. They’ll portray themselves as protectors of Social Security not merely because that might let them peel off some older voters who don’t want to jeopardize their benefits, but also because they want to wage war over the baby boomer voters who are worried that Social Security won’t be there (or won’t be as generous) when they retire.

Not such an easy battle, by the way. The boomers came of age when, for the first time, a majority of Americans were invested in the financial markets. They remember how those markets boomed — though that’s not how that generation got its name — and how in some of those years some of them earned more in the markets than at work. They, like the boomer president, believe in markets and their redemptive powers.

But the Democrats cannot relinquish this group, which they have courted, with modest success, since Gary Hart ran his new-ideas campaign 21 years ago. They have always based their appeal on hoary visions of the future, but this time that might not be the best strategy. Perhaps they will remember that the last generation to believe in a big boom in the markets found itself, only years later, watching a Depression-era president proposing a program called Social Security.

Here are the stakes faced by a party that has lost two consecutive close national elections: If they fail, or if the Republicans blunt their assault, the president who believes in an “Ownership Society” is going to own this decade in American politics.

— David Shribman is executive editor of the Pittsburgh Post-Gazette.