For the first eight years of the decade, we had, in Ronald Reagan, an actor-president. What we now have, in George Bush, is a reactor-president: The more he proves himself highly competent with the comeback, the more maddening is his reluctance to move first.

A returning reporter finds this capital awash in admiration of the energy, acumen and skill Bush has displayed in marshaling a formidable international military-diplomatic-economic response to Saddam Hussein's move against Kuwait. His reaction, one is told repeatedly, has been near-perfect.

That's how it looked in the three Midwest states -- Michigan, Illinois and Indiana -- through which I've been traveling since this Gulf showdown began. And yet the politicians in those states, of both parties, are exceptionally shaky about how all this will play out.

''My concern,'' said one Republican candidate, speaking with a grant of confidentiality, ''is that before Election Day, the question will be not how well Bush reacted but whether we did all that we could have done to prevent the crisis from arising.''

A Democratic campaign manager spelled out the kind of news he thought the voters would be getting in two months, if Saddam Hussein remains in power and continues to menace the world's oil supplies: higher inflation and a spreading slump in jobs and business at home, growing frustration about Americans detained by Iraq, and publicized gripes from and about the thousands of U.S. troops deployed in the desert and on the seas bordering Iraq.

''You know,'' said one former official who served in government during the Vietnam War, ''that if you put that many troops into that kind of environment, there will be incidents, there will be accidents, there will probably be deaths -- even if they haven't come under hostile fire. And the domestic support will begin to chip away.''

That gloomy scenario may be overdrawn, but it explains why one of Bush's former colleagues in government predicts that ''Bush will not allow this to become a protracted engagement. Once he has the pieces in place -- the military forces to contain Saddam and enforce the embargo -- he will rattle Saddam's cage very hard and see if he can't knock him off his perch.'' Already, there have been hints that former CIA chief Bush is ready to encourage a revolt or coup by anti-Saddam elements in Iraq and to exploit what must be the weariness of many Iraqis with the rule of a man who has bankrupted the country and brought thousands of deaths in pursuit of his limitless personal ambition.

Should Saddam fall, Bush would reap the general approval he enjoyed after he responded to a series of increasingly serious provocations from Manuel Noriega by invading Panama and capturing that troublemaker. Once again, the end would have justified the means.

But over time, a reactive president will increasingly be constrained by circumstances not of his making. His leadership will inevitably come to be questioned. To exercise the full range of his constitutional powers, a president must be able to launch and sustain initiatives of his own, not just wait for others' and then figure out how to react.

Bush's reluctance or inability to do that shows vividly at home as well as abroad -- particularly in the matter of the federal budget. Bush reacted to Bob Dole's victory in the 1988 Iowa caucuses by trumpeting the ''no new taxes'' pledge, which prevented his taking the initiative to solve the structural deficit problem in his first year as president. He reacted to the savings-and-loan crisis and the signs of a weakening economy by calling a budget summit three months ago and finally abandoning his unrealistic campaign pledge.

Last week, in his Washington news conference, he complained at length that the Democratic negotiators had failed to propose a budget package of their own, so he would have something else to react to.

He's right. They haven't. They are playing the same politics of procrastination in 1990, when they are running, that Bush played in 1988 when he was running. He was irresponsible then, and they are irresponsible now.

But Bush now is president, and Saddam has given him all the leverage he needs to force the Democrats to act. If he had used his news conference, not to whine about the Democrats' recalcitrance, but to tell the country he was summoning the budget negotiators to Kennebunkport next week and was going to keep them there until they reached an agreement, the deal would have been done in days.

Had Bush demanded action that ''would tell the world -- and our own brave fighting men and women -- that we will back them with a budget that preserves a strong military capability, eliminates the deficit over the next five years and allows the American economy to grow fast enough to meet our domestic needs and our world responsibilities,'' those Democrats would be scrambling to come up with proposals.

Instead, Bush let the negotiations remain stalled until after Labor Day in hopes that the Oct. 1 deadline for the mindless Gramm-Rudman-Hollings across-the-board spending cut will force some desperate improvisation or fictional ''fix.''

He's so good at reacting, you just wish, for once, he would really take the lead.