Most Members of Congress have only a vague sense of what President Bush will propose in the State of the Union speech this evening. But they do know this: Whatever it is, it will dominate this session’s agenda on Capitol Hill.

During the first three years of the Bush presidency, the Republican majorities on Capitol Hill have had to grow acclimated to a world where they no longer call the shots for the party, as they had during the Clinton administration. It’s a world where one speech given in late January by the president will have a more powerful determinative effect in Congress than all the year’s committee hearings.

The agenda is not always as the Republicans would have it. Promises from Bush’s campaign in 2000 have forced the party to pass measures such as campaign finance reform and a prescription drug benefit — items that Congressional Republicans might otherwise have postponed indefinitely.

But that has been the price of unified control of the federal government.

“You’ll get some grousing there,” said Stephen Hess, a Congressional expert at the Brookings Institution. “But you can be very sure that Republicans on Capitol Hill would rather have [Bush] there than a Democrat.”

Bush’s control of Congress has trickled down to even the most basic levels of organization. The House GOP leadership now has a special liaison to the White House — Rep. Rob Portman (Ohio) — while Senate Majority Leader Bill Frist (R-Tenn.) owes his ascension in part to his close personal ties to the president.

Senior GOP leadership aides and appropriations staffers grouse, these days, about being regularly excluded from White House meetings, even as Bush administration aides are welcomed.

The White House has said the exclusions result from lack of space, but Congressional aides suspect a plot: Without Hill staff in the room the White House can coax Members of Congress into making agreements that their aides, who deal in the minutiae, would otherwise advise against.

Capitol Hill staffers involved in the complex business of appropriations say the deals reached at the White House often unravel once Members return to the Hill, because the agreements upset the careful balancing of interests that has already gone into the spending bills. But it’s not as if Members can summon the president to Capitol Hill for meetings.

“Look, it’s their meeting, they can have whoever they want in it,” one frustrated Congressional aide said of the White House confabs. “Not that it’s the right way to do it.”

Whatever the results, such exclusion from the basics of Congressional business has served to underscore a deeper truth about dealings between the White House and Capitol Hill during the Bush administration: This is not quite a relationship of equals.

The centralization of power in the White House has been one of the principal themes of Bush’s first term.

It represents a sharp break from the experience of his predecessor. Faced with Republican Congressional majorities for most of his administration, former President Bill Clinton was often forced to wield power by means of the veto pen — he could affect the agenda insofar as he could prevent the GOP majority from reaching its goals.

Bush, on the other hand, has made Congress an arm of his presidency. In his first three years in office, he has won, among other things, three tax cuts, Medicare reform, “fast-track” trade negotiating authority and an $87 billion supplemental spending bill.

The House Science Committee had held a number of hearings over the years on the future of the space program. But the road map Bush laid out last week for development of a moon colony and a future manned mission to Mars moved the debate beyond academic considerations.

Suddenly, proposals that would have strained the limits of practicality two months ago leapt to the front of the Congressional agenda.

“It takes the president to define the mission,” Rep. Lamar Smith (R-Texas), a member of the Science panel, said last week in suggesting that the president’s support for ambitious new goals in space had immediately changed the nature of the debate. “I think it’s an exciting program.”

Political power naturally shifts to the White House when Congress is controlled by the same party. But Hess suggested that a president’s success under such an arrangement is by no means ensured.

Citing previous presidents such as Andrew Jackson and Lyndon Johnson, Hess said that successful presidents have been those who have been willing to push the limits of their power in their dealings with Capitol Hill.

“This president is certainly in that tradition,” Hess said of Bush. “This is a president who chooses to use to the maximum the powers given to him.”

Hess suggested, perhaps counterintuitively, that the narrowness of the GOP’s margin of control on Capitol Hill actually serves as a source of strength for the president. Not only must he be more forceful if he is to succeed, but Members, who realize their own success depends in good measure on the president’s political strength, are loathe to deliver defeats that might weaken him.

The president’s mastery of Congress has been evident not only in what has occurred on Capitol Hill, but also in what has not.

Single-party control has meant that Congressional oversight is nowhere as aggressive as it was under Clinton, and Bush has not needed to cast a single veto; merely threatening to do so has been sufficient to change the elements of bills that the administration considers undesirable. Often this has occurred in the House-Senate conferences where lawmakers piece together the final versions of legislation.

Bush is now on course to be the first chief executive since James Garfield to not use the veto during a four-year term.

Republicans on Capitol Hill made a determination at the outset of Bush’s presidency that, for better or for worse, their own fortunes would be bound up with the president’s own. The alliance led naturally to pressure GOP leaders to adopt Bush’s priorities.

But many observers — Democrats, in particular — believe the Sept. 11, 2001, terrorist attacks played a key role in sealing the president’s control of the Congressional agenda.

While passing no judgment as to whether the president has been successful in his dealings with Congress, Senate Associate Historian Don Ritchie said the power of presidents has traditionally been enhanced during wartime.

“All [the wars] have helped to pump up presidents to some degree,” Ritchie said. “Certainly, with the war on terror, this president has that.”