President Reagan told Republican senators yesterday he will veto a key component of the Nov. 20 deficit-reduction accord reached with Congress unless legislators scrap what he considers extraneous items or modify other items to make the bill comply with the budget agreement.

The veto threat came as the Senate cleared the way for debate today on a tax and spending measure that will achieve $24 billion of the $30 billion reduction in the anticipated fiscal 1988 deficit promised by the budget agreement.

The measure -- which includes a $9 billion tax increase, savings in federal entitlement programs such as Medicare and farm subsidies, and increased revenues from user fees and asset sales -- is one of two pieces of legislation needed to implement the budget accord. The other is an appropriations measure that the Senate will take up once it passes the tax legislation.

Senior administration officials met sporadically yesterday with Democratic and Republican senators in an effort to make the bill more acceptable to the president.

Among the objections relayed by Treasury Secretary James A. Baker III and budget director James C. Miller III was one to a provision that would enact the "fairness doctrine," under which the government used to required broadcasters to air opposing views on controversial issues. Reagan has already vetoed separate legislation to make the standard law.

The two administration representatives also argued that the agricultural subsidy provisions would not produce the promised $900 million in savings and that the measure would create a new loan program for soybeans in 1990. They also sought to scale back by half a provision in the bill permitting refinancing of $5 billion in rural electrification loans and replace the lost savings with other sales of federal assets.

According to one senior administration official, Reagan told four Republican senators yesterday afternoon that "if it came down to Christmas Eve and he got a bill with all these ornaments on it, that he wouldn't hesitate to veto it."

One senator who met with Reagan, Pete V. Domenici (R-N.M.), said he urged the president to "stand tough" by threatening a veto in order to make sure the offending provisions are removed in a House-Senate conference committee that will follow Senate passage of the bill.

Senate Minority Leader Robert J. Dole (R-Kan.) said last night that the bill may not pass the Senate. Dole, campaigning for the Republican presidential nomination in Manchester, N.H., said, "We're not even certain we can pass that little baby package . . . because people are starting to back out."

An amendment introduced last night by the Senate Democratic leadership reduces some, but not all, of Reagan's concerns.

The $9 billion in higher taxes called for in the measure would be raised from a multitude of changes in corporate and individual taxes, most of which would not affect ordinary taxpayers. All but two of the proposals are also included in separate tax legislation previously adopted by the House, although the House bill has provisions not included in the Senate version.

One provision that affects most households is a three-year extension of the 3 percent tax on telephone bills, which would raise $1.3 billion. Some tax changes are:Closing an estate tax law loophole that has permitted some estates to lower their taxes by selling stock to an employee stock ownership plan. The provision would raise $1.2 billion. Eliminating an accounting method used by some manufacturers and real estate dealers that sell on installment, which would raise about $2 billion. Extending by three years the current federal unemployment tax rate of 6.2 percent paid by employers on the first $7,000 in wages collected by employes, which would raise about $715 million.