Both Maine Sens. Olympia Snowe and Susan Collins understand that elimination of the estate tax is unaffordable in the face of a rising national debt and the expected costs of the war, Medicare, Social Security and the alternative minimum tax exemption, to name a few. But opposition to full repeal isn’t enough because Congress is determined to pass an extension to estate-tax exemptions before the next election. Finding an affordable compromise is crucial.

The Senate today is likely to consider a motion to bring an estate-tax repeal to the Senate floor. Knowing that full repeal is unaffordable, Maine’s senators should reject this motion unless realistic compromises will also be offered and debated.

Like so many Bush administration tax cuts, the current exemption to the estate tax – “death tax” for those who want to make it sound scary – is temporary to hide its true cost. But no one believes that when it expires, in 2011, the tax will be allowed to return to the old standard of exempting only the first $1 million of an estate. Sen. Jon Kyl, a Republican of Arizona, has the leading alternative, which would set the exemption at $5 million for an individual ($10 million for a couple) with the top tax rate at 15 percent, down from about 40 percent.

The Joint Committee on Taxation has concluded that total elimination of the estate tax would cost $369 billion in lost revenues between 2007-2016, though that doesn’t count the added interest on the debt nor the fact that the repeal would not start until the second five years and so is really measuring less than half the cost. Either way, the repeal is unaffordable, and the Kyl proposal, which would reduce revenues by about 84 percent of total repeal, is almost as pricey.

A real compromise is possible, starting with a higher threshold for when the tax begins. In 2009, for instance, that threshold will be $3.5 million for individuals and $7 million for couples. Automatically adjust that for inflation and virtually all estates will be exempted. But Congress should go slower when adjusting the tax rate above that threshold because whatever cut Congress chooses comes with an obligation – the costs facing the federal government are undeniable – to make up for the lost revenue. Yet all of the alternative sources (income tax, payroll taxes, a broad national sales tax?) are worse.

Program cuts are the next choice, but which senators want to provide the beneficiaries of the wealthy with a tax break offset by cuts to Medicare coverage? Debate on the estate tax was delayed from last year because Congress was fearful of eliminating the tax while the victims of Katrina were in desperate need of aid. The coming demands on entitlement programs will make Katrina expenditures seem puny.

Congress could easily do nothing with the estate tax for several years, but the political reality is that it is determined to act now. Sens. Snowe and Collins should act as responsibly as possible given the nation’s upcoming challenges.