Senators trying to slash President Reagan's "Star Wars" missile defense budget picked up a near majority of the Senate yesterday as Bill Bradley (D-N.J.) and Orrin G. Hatch (R-Utah) became the 47th and 48th senators to state in writing that they think the Strategic Defense Initiative is getting too much too soon.

The group is pressing the Senate Armed Services Committee to reduce SDI funding from Reagan's requested $4.8 billion for fiscal 1987 to $3 billion. (If Energy Department funds for SDI are included, the total request is $5.4 billion.)

Jumping from this year's level of $2.76 billion to $4.8 billion would give SDI "excessive and inappropriate emphasis" in the Pentagon budget at a time when SDI's goals are unclear and when "the need for accelerated funding has not been demonstrated," the senators said in their letter to the panel.

In the first SDI test of the year, an Armed Services subcommittee voted 6 to 5 Thursday to cut the program about 10 percent, while halving Reagan's $1.4 billion request for the Midgetman missile. A bigger battle on SDI will come next week when the full committee marks up the Pentagon's fiscal 1987 budget.

Senators preparing the case against SDI have consulted military leaders about their priorities down the road and, according to aides, have found little enthusiasm for spending as much money as Reagan is requesting for the missile research effort.

The Pentagon's counterattack features the argument that Congress has cut deeply into the SDI budget and that additional cuts will make it difficult to explore promising technologies to the point that a sound decision can be made on the feasibility of a missile defense.

An alliance of 100 national organizations has focused on SDI in seeking a highly visible victory in trying to cut the fiscal 1987 Pentagon budget. One of those organizations, the self-styled citizens lobby Common Cause, recently sent a letter to members of Congress claiming that SDI "is a runaway program" that, "if pursued, will trigger a new round in the arms race."