Undeterred by charges of "obstructionism," Senate Republicans have broadened their horizons, reaching out to thwart their Democratic colleagues' initiatives from the House as well as the Senate.

After stalling Senate Democratic moves on issues ranging from campaign finance to the Antiballistic Missile (ABM) Treaty, GOP senators have served notice that they will block a catchall spending compromise for the remainder of this fiscal year as long as it includes arms control language pushed by the House.

Because the nearly $10 billion supplemental appropriations bill for fiscal 1987 includes money urgently needed to resume payment of farm subsidies and crop loans, pressure has built on House Democratic leaders to abandon their insistence that the Senate vote on at least one of the nuclear arms constraints.

They have already backed off a proposed ban on nuclear testing and lowered their sights on the SALT II strategic arms limitation treaty, urging only that the Senate vote on the House proposal to require that the Reagan administration resume compliance with weapons limits in the unratified pact, which the administration abandoned last year.

But Senate Republicans, fearing that the SALT II language might muster a majority in the Senate now that Democrats have taken control of the chamber, have served notice that they would block action on the whole spending bill if even this requirement is included.

They could do so by exploiting Senate rules that require "super-majorities," such as 60 votes to break a filibuster or to pass a waiver to allow consideration of spending bills that exceed budget ceilings, a category that includes the fiscal 1987 spending bill.

With Democrats controlling the Senate by a ratio of only 54 to 46, Democrats must rely on GOP defections to get around these hurdles. So far the Republicans have held ranks sufficiently to thwart the Democrats, and there is every indication they will do so again on the supplemental spending bill. So Democrats are trying to work out an agreement under which they would abandon any SALT II provision in the spending bill in return for assurances that treaty compliance will get a vote in connection with other legislation, such as the defense authorization bill.