President Reagan, battling with Congress over funds for his "Star Wars" system, on Saturday credited the controversial program with forcing the Soviet Union into arms reduction talks.

Reagan earlier in the week vetoed a defense spending bill, in part because it fell short of the $4.8 billion he sought in funding for the space-based system formally known as the Strategic Defense Initiative."If I had accepted that bill, it would have undermined the (military) strength we worked so hard to restore, in time jeopardizing all our remarkable diplomatic advances," Reagan said in his weekly radio address.

"Congress' defense bill represented an all but open attempt to block our Strategic Defense Initiative," he said.

"Yet no development has been of greater importance in our strategic arms negotiations with the Soviets than our decision to proceed with SDI.

"Congress wants to cut our budget request for SDI deeply," Reagan complained. He said the bill "represents an attempt by Congress to handcuff the president in our arms reduction negotiations with the Soviets."

"This kind of unilateral concession to the Soviets could set back all the progress we've made on the arms reduction front," he said.

The Senate on Friday rejected an attempt to add $500 million for the SDI program to a revised defense bill, which would authorize $4 billion for the project in the next fiscal year.

Sen. Sam Nunn of Georgia, in the Democrats' response, accused the president of pandering to election-year politics in his veto of the bill, hoping to aid Vice President George Bush.

"I had hoped that President Reagan would put national defense above partisan politics," said Nunn, chairman of the Senate Armed Services Committee.

"I had hoped that President Reagan would listen to those who urged him not to veto this important bill, including his secretary of defense and his national security adviser," Nunn said.

"Unfortunately, the president followed the advice, not of his national security team but rather of Vice President Bush's political team," Nunn charged.

Bush has been attacking the Democrats as weak on defense and cites the defense bill as an example.