President Bush's dogged pursuit of a capital-gains tax cut in a time of war and rising deficits is one of the enduring mysteries of Washington. Perhaps its inclusion in the State of the Union address, especially in the context of a new "study" by a commission headed by Federal Reserve Chairman Alan Greenspan, was intended as a sop to the GOP's right wing.

Many on Capitol Hill thought it had been scrapped in a tacit agreement between the White House and Democratic leaders to dump discussion of both capital-gains tax cuts and income-tax boosts on millionaires.

Even Treasury Secretary Nicholas F. Brady, supported by his own analysts, had thought it unwise to revive special treatment for capital gains at this time. But he was overruled by a majority of other Bush advisers.

"As a Democrat," says a well-known economist, "I can only welcome self-inflicted injury by the Republicans. Why does Bush insist on atrocious policy and even worse politics?"

Bush's decision to tap Greenspan for the job of assessing the capital-gains question is a clear signal he is prepared to ask Greenspan to stay at his post after his current term expires at midyear. Although the White House had been impatient with Greenspan's slowness to lower interest rates in 1990, recent Fed actions in that direction have substantially eased the tensions that existed before.

Insiders say Bush is committed philosophically to the idea that a capital-gains tax cut will stimulate the kind of entrepreneurial investments that trigger growth -- an assumption that stirs bitter dispute. In last year's budget, he proposed excluding from taxes 30 percent of gains on assets held for three or more years, with smaller exclusions for assets held for shorter periods.

Opponents suggest that not only would most of the benefits go to the richest taxpayers, but the change would undo one of the great achievements of the 1986 reform act by re-creating the tax-loophole industry.

Proponents argue that taxing capital gains at nearly the same rate as normal income discourages the sales of assets and tends to lock in investors who otherwise might finance risky ventures. They point out that in Europe capital gains aren't even considered as income.

In Bush's address Tuesday, he acknowledged "differences among us about the impact and effects of a capital-gains incentive." He designated Greenspan to head a commission, with congressional representation, "to sort out our technical differences so that we can avoid a return to unproductive partisan bickering."

Obviously, Bush hopes for a replay of the success of the Greenspan commission that in 1983 led to reforms putting the Social Security system on a more sound actuarial basis. But Greenspan's function then was to achieve a political compromise on how to achieve economic changes that were widely accepted.

This is different: Greenspan is being asked, in effect, to review diametrically opposite views among economists over the results and accuracies of their econometric models and projections. The Fed chairman himself favors lower capital-gains taxes, or none at all. He could be expected to try to maintain a judicial posture as chairman of a commission. But there is no reason to think that Greenspan is ideally suited as an economist to resolve technical arguments on the likely results of a capital-gains tax cut.

Last August, the Congressional Budget Office analyzed eight different econometric studies of a 30 percent capital-gains exclusion, including two of its own and one by a team whose members came from the Treasury and the Council of Economic Advisers. The CBO's net judgment was against a capital-gains reduction. CBO Director Robert Reischauer reiterated to the House Budget Committee as recently as Wednesday that he remains "skeptical" that such a tax cut would benefit the economy.

In its August report, CBO said that of the eight studies, five -- including its two -- "found that cutting taxes on capital gains is not likely to increase saving, investment and GNP much if at all." It concluded that the other three -- including the Bush administration study -- had made optimistic assumptions "at the high or outside of the range of most empirical evidence."

CEA Chairman Michael J. Boskin, a leading advocate of capital-gains tax cuts, had not responded until now to the CBO charges. But in a conversation with The Washington Post, he rejected the CBO's conclusions and faulted the quality of the watchdog agency's analysis. "They {CBO} presented an extremely inaccurate statement of what we did," Boskin said. "At each and every step, our staffs used the middle of the range of professional opinion on the subject." A detailed CEA rebuttal challenging the professionalism of the CBO's August study is to be released next week.

So it's clear that the already heated debate on capital gains won't cool down. I doubt that a Greenspan commission will generate anything to shift the preponderant Republican view that a cut in capital-gains taxation or, at minimum, some adjustment for inflation, will help the economy. The Democratic view -- that favored capital-gains treatment will help the rich cut their taxes and do little, if anything, for economic growth -- is also not likely to be altered.

At the least, a commission approach ought to take the issue off the congressional agenda for this session. But with all of the demands on the time of the Fed chairman, I wonder if the exercise is worth the trouble.