The word is out along the corridors of power on K Street: the Democrats can retake the Senate in 1986. A trickle of speculation by political observers has become a torrent of near certainty. The pieces of the pattern of the "six-year itch" -- stiff repudiation of the president's party in the off-year elections -- seem to be falling in place. A possible Democratic victory has now become the conventional wisdom. Business and interest groups are already shaping their strategies and calculating their tactics in anticipation that the Senate will turn over. Never mind that conventional wisdom is usually dead wrong (this time two years ago, it was said Walter Mondale was absolutely invincible in the primaries and would give the president a close race); it is shaping perspective and judgment today.

But what if the Democrats actually do win control of the Senate next November? In an immediate sense, the answer is clear: President Reagan would be deprived of his congressional base of support. His policies would be fatally compromised. With control of both houses of Congress, the Democrats could legitimately claim as forceful a mandate to govern as the president's. The new Senate leadership, united with their House colleagues, would be far less responsive to the administration's policies and priorities.

Congress would become equal partners with the president on all issues -- budget, taxes, foreign policy. There would be far less deference on executive branch appointments and particularly on any Supreme Court vacancies. The cement would really have to crack around the president's feet, with a much higher premium on consensus, if he were to enjoy any significant measure of legislative success in the last two years of a lame-duck term. The alternative would be a repeat of our experience the last time the White House and Congress were divided on a partisan basis: government by veto. In 1974-76, President Ford, unable or unwilling to reach an effective accommodation with Congress, vetoed 48 bills and was overridden 12 times -- the most in recent memory.

But the larger question the Democrats must confront as they anticipate regaining substantial national power is far more significant: is the party ready to govern? Regaining the Senate would legitimize the notion that the Democrats can come back and win the presidency in 1988. But can the reality of the Democrats in power in Congress meet such political expectations? Do the Democrats have an agenda for the economy, for social programs, for the budget, taxes, international affairs, strategic arms? More important, does the country believe there is a cohesive Democratic program on inflation, economic growth and trade?

The Democrats do not have the luxury of time. These issues are being fought out -- today -- on Capitol Hill, and they are fraught with peril. The Gramm/Rudman/Hollings balanced budget proposal, sprung so cunningly in the Senate, can, if fumbled, deprive the Democrats of their most potent issue: the massive Reagan deficits. Similarly, trade legislation, if mishandled, could saddle the party again with the "special interest" label. The short-term political benefits of saving several thousand jobs in vulnerable industries could be outweighed by the high, long-term costs of angry consumers facing renewed inflation caused by protectionism, coupled with the loss of hundreds of thousands of jobs if legislation triggers retaliation from abroad. The temptation to vaporize "Star Wars" could backfire if Democrats are seen as preventing SDI from being effectively deployed as a bargaining chip with the Soviets.

The Democrats, in short, have got to get it togethe for 1986, because it will take a constructive and cohesive program to cement a victory in 1988. The leadership is there throughout Congress, and the fertile seeds of pragmatic policies are also there.

It is the Democrats in Congress who have fought for economic sanity on taxes as one tool to help curb the deficit; who have made the administration face up to the trade crisis and developed the novel concept of access to foreign markets as opposed to erecting barriers to our markets; who have tried to manage the defense buildup, rather than simply throwing money at the Pentagon; who want rational arms control with the Soviets; and who have not turned their backs on the farmers.

But there remain severe divisions in the party. The Senate Democrats were bitterly and evenly split on the balanced budget amendment vote. The leaders in the party have yet to make clear to the American people that they will not be dominated by discrete constituencies with rigid demands of little relevance to the middle class, where elections are fought and won. Democrats are not yet positively associated in the minds of voters with policies that will guarantee economic growth and low inflation.

The Democrats' juices are flowing, however. State by state, race by race, the outlines of a Democratic victory in 1986 can be clearly seen. The larger questions tend to get shunted aside in the endless polling, fundraising, media advertising and campaigning. But it would be a shame if the Democrats woke up next Nov. 5, with the Senate in hand, only to echo Robert Redford's coda in "The Candidate": "What do we do now?"