The conventional wisdom is that George Bush was just blowing smoke when he promised not to raise taxes, but there is growing evidence that this could turn out to be his strongest economic issue of the campaign.

And among those who seem to recognize this is Michael Dukakis, who is scrambling now not to mock Bush on this question but, somewhat improbably, to join him.The latest evidence that the vice president has finally found an effective economic message comes in a poll conducted by the Roper organization for the TV special, "Louis Rukeyser's 1988 Election Guide." While the overall results were not favorable for Bush - they showed Dukakis pulling ahead among likely voters, 49 percent to 43 percent - there was a vivid exception on the tax question.

The poll found most voters believe Bush's flat pledge not to raise taxes. Only 41 percent said they expect the taxes they pay to be higher if Bush is elected, whereas a majority - 52 percent - expect to be hit with a higher tax bill if Dukakis wins.

What's more, a solid 55 percent - and an even higher proportion of people who now intend to vote for Dukakis - think U.S. federal taxes are already too high.

Such findings clearly have the Dukakis camp worried. Dukakis' designated economic spokesman on the program was Harvard's Professor Robert Reich, scarcely known before now as an advocate of tax restraint, but you might have thought you were talking with Milton Friedman when Reich gave the latest Democratic thinking on taxes.

Gone was the old Dukakis emphasis that no responsible president could rule out a tax increase - an approach that smelled of a man just itching to introduce one.

Instead, Reich described his candidate as a man almost repelled by such a prospect. "Michael Dukakis has pledged himself that, if he is going to even look at taxes, it's going to be as a last resort," Reich said, adding that Dukakis as Massachusetts governor had "lowered taxes seven times."

This vision of Dukakis as a sort of latter-day Howard Jarvis was promptly challenged by Bush's man, New Hampshire Gov. John H. Sununu, who said: "The reason the voters out there think Michael Dukakis is going to raise taxes is that he has a history of doing that."

But the point here is not to assess the partisan claims on both sides, but to note the new Dukakis stress on tax restraint - as opposed to the emphasis on expanded spending programs that dominated his primary campaigns. The Jesse Jackson agenda seems to be receding daily, as the tax issue begins to bite for Bush.

This is not the first time this year that a pledge to hold the line on taxes has emerged as political gold for the Vice President. After the Iowa caucuses, in which he was thrashed by Bob Dole, Bush turned the tables in the New Hampshire primary by stressing (with Sununu's help) that he would not raise taxes, whereas Dole's Senate record suggested that he probably would.

After we acknowledge the oft-underrated electoral potency of such a pledge, two legitimate questions remain: Is it a pledge that should be kept? and Is it a pledge that will be kept?

On the first, the evidence seems persuasive that U.S. taxes are plenty high today. The nonpartisan Tax Foundation reports that the much-touted "tax cuts" of the 1980s have evaporated, and that the combined federal, state and local burden on the average American has just reached an all time peak.

On the second question, skepticism is justified. While there is no evidence that raising taxes reduces deficits (indeed, Congress generally feels compelled to spend, at a minimum, all the new revenues plus 25 percent), neither side has a wonderful record at permanently restraining either half of the budget equation.

But the increasing bipartisan recognition that tax increases are particularly abhorrent right now alters at least the short-term odds. If Bush is elected, and now perhaps even if he isn't, the 28 percent top rate on income is likely to be left alone by Congress in 1989. That's a change from the expectations even a few months ago, and it's not all bad.