With debate over the Comprehensive Test Ban Treaty scheduled to start today, the Senate is engaged in three separate--but intersecting--struggles as it moves toward a ratification vote next week that few of its members seem to want.

First is the argument over pros and cons of the treaty, which would ban underground nuclear tests as well as atmospheric and other explosions outlawed by earlier treaties.

Second is the effort by many senior lawmakers to cancel the vote on grounds that it would almost certainly result in the treaty's rejection, setting off a potentially dangerous political, diplomatic and even military chain reaction.

Third is the politics that seems to drive everything else on Capitol Hill this year: worse-than-ever relations between President Clinton and Congress's Republican majority in the wake of Clinton's impeachment and trial last winter.

Important as debate over the treaty itself may be, it is politics that helps explain why both sides have had such difficulty reaching agreement on how to deal with the pact.

As Republicans see it, Clinton dawdled over the treaty for two years, then joined with Senate Democrats to push for action for political reasons--a "legacy trip" to ensure a major foreign policy victory before he leaves office 15 months from now, Sen. Pat Roberts (R-Kan.) suggested earlier this week.

Other Republicans, long suspicious of Clinton's motives on many issues, contend that he and his Democratic allies promoted a flawed treaty so they could portray treaty foes as latter-day Dr. Strangeloves.

As Democrats see it, Republicans decided to call Clinton's bluff by scheduling a vote next week because they are frustrated with his legislative Houdini acts, escaping their clutches on issue after issue and hoping to do so again in a showdown over spending priorities later this month.

"It's an understandable frustration that they've not fared very well in their legislative battles with this president over the past eight years," Minority Leader Thomas A. Daschle (D-S.D.) said yesterday. "So maybe it's payback time."

The effort to find a graceful way to back off the vote, which was still underway yesterday, arises out of both political and policy concerns.

Democrats argue that Republicans are courting a backlash at the polls if they vote against the treaty, especially if there is some nuclear-related incident before the elections, and some Republicans also appear concerned about political fallout. At the same time, Clinton and the Democrats do not want the embarrassment of a flat-out defeat on one of their biggest foreign policy initiatives.

Many Republicans as well as Democrats also worry about the shock effect of U.S. rejection of the treaty on other countries, including some that might be persuaded to swear off testing if this country leads the way. Some are disturbed that a precipitous vote along party lines could make it more difficult to reestablish bipartisanship on arms control in Congress.

As of late yesterday, Majority Leader Trent Lott (R-Miss.) was still insisting that Clinton agree not to resurrect the treaty before he leaves office, a condition rejected by the White House. And some conservative Republicans vowed to block the unanimous consent that would be needed to cancel the vote, although Democrats said there appear to be parliamentary ways to get around their objection by majority vote.

A bipartisan group was trying to work out a way to avoid a vote, but Sen. Joseph R. Biden Jr. (Del.), ranking Democrat on the Foreign Relations Committee, said he was "not hopeful."

About all the two sides agree on is that, at this point, the treaty will be defeated if it goes to a vote as scheduled next Tuesday or Wednesday. Proponents concede they have nowhere near the two-third majority required for ratification. Democratic leaders have said they expect all Democrats to vote for the pact, but only two Republicans have said publicly they support it--leaving the treaty 20 votes short of the required 67. Yesterday, Sen. Richard G. Lugar (R-Ind.), an influential arms control advocate, came out against the treaty.

Both Clinton and Secretary of State Madeleine K. Albright stepped up pressure for ratification yesterday.

"This is bigger than personal politics. This is about America's future and the future of our children and the world," Clinton said, adding that "future generations won't forgive us" for failure to ensure a more secure world.

Testifying before the Foreign Relations Committee, Albright said that while no treaty could guarantee to stop the spread of nuclear weapons, the test ban pact "will make it more difficult and dangerous for countries to develop and modernize nuclear weapons." With the United States having stopped testing in 1992, the treaty "is about preventing and dissuading others from doing so."