THE HISTORIC Republican takeover of Virginia's state government became official yesterday as new leaders took charge of the General Assembly in the richest of times. With Republican Gov. Jim Gilmore setting the tone in the only two-year budget that he can call his own, GOP governance began smoothly. But if budget surpluses make this a pleasant time to govern, the balance of partisan power in the legislature remains delicate; regional alliances could spell the difference on major issues such as roads, schools and health care.

Top priority for Northern Virginia's 34 lawmakers, working in bipartisan alliance and in coalitions with other regions, should be transportation. Gov. Gilmore's six-year, $2.5 billion plan for roads and transit is only a start. Northern Virginia is staring at road and transit needs that require more money than Gov. Gilmore has in mind. Representing 25 percent of the state's residents and a mighty nucleus of high-tech firms contributing a major portion of state revenues, Northern Virginia's delegation should have clout on this issue.

One valuable tool would be a regional authority similar to the Metro partnership. Major bridge and highway projects don't all stop at state lines; too often, they don't start anywhere, because local and state governments don't work together. The idea is not to create a supergovernment but to establish a federal-state-local coalition that, like Metro, could settle on projects and build them with funds from participating governments. The Virginia and Maryland legislatures, which both opened yesterday, ought to move matching bills.

Gov. Gilmore's favorite financial project--the issue that won him election two years ago--is the repeal of the car tax. Forgone revenue reaches nearly $1 billion a year in his biennial $48 billion budget. The car tax repeal has proven too popular for any lawmaker to challenge, and boom times have eliminated any pain so far. But responsible lawmakers should resist any more mortgaging of the future in the name of tax-repeal pledges. If anything, the legislature should accelerate cuts in the 4.5 percent grocery tax, which hits poor people hardest. At this point, cuts of one-half a percentage point are scheduled for April of 2001, 2002 and 2003. (The first cut went into effect Jan. 1.)

Gov. Gilmore and Republican legislative leaders are pledging cooperation with Democrats. The new House Speaker, S. Vance Wilkins Jr., has been traveling across the state seeking to bridge regional gaps, speaking with minority groups as well as business and civic organizations. Now the test begins: How well will the new management govern?