I've reported here repeatedly that the Democratic Party is riding a long-term wave of demographic and cultural trends that bode well for its future.

A more urban, multi-ethnic, women-empowered society — and those are all measurable, real-world changes our nation is experiencing right now — will almost certainly benefit the party of Obama and Pelosi.

But as we head into the crucible of the 2012 election, there is still a massive, gaping omission in the story that the Democrats are telling to voters, one they will need to remedy if they are to become the party of the future.

Put simply, Democrats need to explain how they will pay for the government which they believe America wants and needs.

Before I explain what I mean, let me detour for a moment to point out that Democrats don't need to spend much time or energy arguing in favor of their vision of "big" government.

By overwhelming margins, Americans support all the big-ticket items that make up about 90% of the US budget, from Social Security to Medicare and Medicaid, to education, the military and homeland security.

Yes, we all grumble about pork and waste.  But that's just the normal bird-dogging of citizens who, quite reasonably, want to get good value for our tax dollars.

There is no evidence that voters have bought into the broader, conservative, Ron-Paul-esque notion that the fabric of government itself needs to be unraveled or dismantled.

When pressed, Americans are even pretty comfortable with the idea that there should be an appropriate safety net, to help citizens who stumble, or fall into poverty, especially if they are children or senior citizens.

And we also want — indeed, we demand — a robust network of police and first responders.

The big question, then, isn't what government should look like in the future.  The real question — and, yes, I lay this predominately at the feet of Democrats — is how to pay for it.  How to sustain it over the long term.

Currently, roughly half of all US spending is borrowed.  Which means that any vision for a long-term, stable government on the scale that Democrats (and their constituents) want will have to include some enormous changes.

Some cherished services will almost certainly have to be cut, not because we oppose them ideologically but because they are just too expensive.

I'm guessing that in the future people probably won't be able to retire at age 65 and draw government checks for the next quarter century.

Other services will have to be provided more cheaply, either by allowing the private sector to deliver them (not always the solution, but in some cases it will help) or by demanding concessions from public employees.

(The era of lifetime health insurance and pensions ended long ago for private sector workers, and I'm betting the time has come for public sector workers to see a big change as well.)

We will also have to generate a lot more revenue.  Some of that will come from growth, as the economy bounces back, but it's also time to level with the American people:  all of us will have to pay more if this is really the government we want.

Taxing rich people won't get us there.

The short-term reality, of course, is that Republicans will block enactment of any vision that achieves a sustainable balance.  They'll argue that even when balanced with spending cuts, any new tax revenues are a socialist scourge.

But that doesn't mean Democrats can't or shouldn't lay out what their plan looks like.

On the contrary, that vision should be the cornerstone of an honest campaign, both for Mr. Obama and for Democrats running for congress.

Some on the left will point out that Republicans have also quietly embraced big government, and done little to bring down our national debt.  This is true.

Most economists believe the various budgets put forward by GOP leaders over the last year would grow rather than shrink the long-term deficit, because of massive tax cuts that aren't off-set by spending cuts, and because of plans to grow the military.

But fair or not, the identity and core values of the Republican Party aren't linked to the health, quality, and sustainability of the Federal government.

On the contrary.  Many conservatives would be quite cheerful seeing even good programs cut or eliminated, even if it requires insolvency to get us there.

So for better or worse, Democrats carry the torch of the government model created during the New Deal.  They will be the ones to figure out how to pay for it, and put it on an even keel, or no one will.

Until President Obama, Senate Majority Leader Harry Reid, and Minority Leader Nancy Pelosi put forward that vision, they will remain vulnerable to the suggestion that their vision, no matter how laudable or popular, is simply a pipe dream.

And right now, that pipe dream is adding about $1 trillion a year to the national debt.

As always, your comments welcome.

The next half-year, Republicans and Democrats will churn out a lot of words, wrangling over everything from deficits to global warming. And in theory, elections should be about issues.

But one fascinating element of this year's campaign is how distinctly tribal it appears to be. We are, despite our collective myths and ideals, a society of distinct and very different ethnic and cultural groups.

And we see the world very differently.

One of the latest polls, from the Christian Science Monitor gets at this reality starkly.

It reveals that while Barack Obama and Mitt Romney are essentially tied, the two men are incredibly polarizing figures when looked at from the perspective of our various "identity enclaves."

Obama is winning just 37% of the white vote, compared with 86% of the black and Hispanic vote. That's a see-saw pattern with profound implications.

Romney, meanwhile, is losing badly among women (40% to Obama's 49%) but he absolutely dominates among white women (51% to 39%).

Another dynamic that shapes the race pits our urban tribe (61% for Obama) against our rural tribe (51% for Romney).

And the nation's regions are also dramatically different, with the Northeast and the West solidly in Obama's camp and the Midwest and the South tilting toward Romney.

Obviously, a lot of these cultural dynamics overlap.

The South and the Midwest are more rural than the rest of the US, and many small towns tend to be whiter than urban and suburban areas.

One other remarkable thing is that with all of these patchwork-quilt differences, we still add up to a roughly 50-50 divided nation.

It may be that in the future the rise of the Hispanic population and increasing urbanization will move us significantly in the Democrats' direction, as some pundits have suggested.

But for this election cycle, and the near-future, our various tribes have formed up into something like a stand-off.

Whoever wins in November, it appears that roughly half of the population will leave the polls dissatisfied.

Indeed, some of the factions that make up our society will be deeply grieved by the outcome, contributing to the sense that America is a democracy struggling to find its center.