As President Obama is finding out, spending a trillion dollars on health care reform is easy; paying for it is a bit harder.

Both the House and Senate versions contain huge tax increases. But they take completely different approaches toward which taxes are hiked and who would pay them. And, as President Obama discovered in yesterday’s contentious meeting with labor bosses, those differences will not be easy to resolve.

The Senate wants to slap a 40 percent excise tax on so-called “Cadillac” insurance plans, that is plans with an actuarial value of more than $8,500 for an individual and $23,000 for a family. The tax technically falls on the insurance company that offers the plan, but there’s widespread recognition that insurers will merely pass that tax on to their customers in the form of still-higher premiums. The Congressional Budget Office estimates that initially about 19 percent of insurance plans would be subject to the tax, and union surveys suggest that it could hit as many as 25 percent of union workers. Moreover, as inflation drives costs higher, more and more plans will be subject to the tax. That is because the threshold for the tax is indexed to general inflation not medical inflation which runs higher.

As today’s Washington Post editorial points out, economists and deficit hawks see this measure as one of the few cost-control provisions left in the bill. Its goal is not just to raise some $150 billion in revenue over 10 years, but to discourage the type of “gold plated” insurance plans that encourage over utilization and drive up costs. That is why the Obama administration has endorsed this approach.

However, as labor leaders made clear in yesterday’s meeting with the president, this middle-class tax hike is unacceptable. AFL-CIO president Richard Trumka has even threatened to retaliate at the polls against Democrats who vote for it. In addition, 124 House Democrats have signed a letter opposing the “Cadillac tax.” With just a three vote margin, House Speaker Nancy Pelosi cannot afford to have any defections from tax opponents.

The House, on the other hand, has gone with a “soak the rich” strategy, calling for a surtax on incomes of $500,000 or more a year. But Democrats already plan to allow the Bush tax cuts to expire next year, raising income taxes for millions of Americans. An income tax surtax on top of that would mean marginal tax rates of more than 50 percent in many states with devastating consequences for economic growth. Moderate Democratic Senators like Ben Nelson (Neb.) and even liberals from states with high cost of living like Chuck Schumer (NY) are unlikely to go along with this tax. And, in the Senate, Democrats can’t afford even a single “no” vote.

The conventional wisdom in Washington is that a health care bill is inevitable. But if the growing fight over taxes is any indication, inevitability is overrated.