With a new series of television ads that could be titled "The Red Phone Meets Darth Vader," Democratic presidential nominee Walter F. Mondale has launched a video game-styled assault on President Reagan's "Star Wars" plan to put defensive weapons in space.

And Reagan is ready to return the television fire, as his strategists have prepared their first national security ads of the year, a series of commercials that take Reagan well beyond his early weeks of Dr. Feelgood ads but stop short of Dr. Strangelove.

One new Reagan video is a barbed remembrance of the Carter-Mondale administration's handling of the Iran hostage crisis, the issue that, more than any other, helped make Reagan what he is today.

Another pushes the importance of being strong enough to wrestle the Soviet bear.

After weeks of Mondale and Reagan television commercials focused on domestic issues -- mainly national debt versus national prosperity -- both campaigns have switched to 30-second snippets of international affairs as a prelude to Sunday's televised presidential debate on foreign policy.

Mondale's ads opened first, on Tuesday, as his campaign strategists sought to invoke fear-and-loathing in outer space.

From somewhere in space, the earth is seen against a black sky, as an announcer says: "It's from up here that President Reagan, if reelected, is determined to orbit killer weapons. He'll spend a trillion dollars. The Russians will have to match us. And the arms race will rage out of control -- layer on layer -- orbiting, aiming, waiting. Walter Mondale will draw the line at the heavens. No weapons in space -- from either side. On November 6, draw that line with him."

On Thursday, Mondale officials fired their second stage: a Mondale ad featuring the familiar red telephone that Mondale officials used during the primary season to create doubt about Sen. Gary Hart's steadiness in crisis. Only this time there are some computerlike buttons attached to the red phone. And this time, the real enemy is depicted as time.

The red phone is blinking as the announcer begins: "Ronald Reagan is determined to put killer weapons in space. The Soviets will have to match us. And the arms race will rage out of control -- orbiting, aiming, waiting."

The pulsating warnings quicken and as the camera moves from the red phone to the computer that controls it, the announcer warns: "With response time so short, there'll be no time to wake a president. Computers will take control. On November 6, you can take control. No weapons in space -- by either side. Draw the line at the heavens -- with Mondale."

The third stage of the new Mondale media rocket is Mondale. He is shown in the very unfuturistic style of simply talking into a camera, saying the danger of Reagan's nuclear arms plan is that "human beings may no longer control our destiny. Computers could make humanity's ultimate decision."

The ads, the work of media adviser Roy Spence, are airing as polls are showing that Mondale has achieved only minimal gains from his weeks of hammering at Reagan's record budget deficits, his views on Social Security and Medicare. The new ads, Mondale officials said, are targeted at converting 13 percent of the Reagan-leaning voters who they contend have doubts about Reagan and are "movable."

"We're moving into foreign policy and especially arms control because we think that's where he Reagan should be most vulnerable," said a senior Mondale strategist. ". . . We've got to get people to burst out of this bubble that the Reagan campaign has created. People have got to look beyond the white picket fence. So we took them into outer space."

Reagan officials scored well with their initial series of low-key, soft-sell ads crafted to make people feel good about America and sense a return to prosperity. In recent days, Reagan ads took a harder line, attacking "Mondalenomics" as being only a plan to "raise taxes" and presenting a series of vignettes, in which a narrator tells working people that "Walter Mondale thinks. . ." that if they will just work harder they can pay for his new taxes.

Now Reagan strategists believe the national security issue can be turned to Reagan's advantage. Thus, they say, they plan to air their reminder of the Iran hostage crisis of the Carter era, the year that ABC News portrayed nightly as "America held hostage."

And tonight, on the eve of the presidential debate on foreign policy, Reagan strategists say they will air their favorite peace-through-strength ad.

It features a large, menacing brown bear walking in the wilds as an announcer says there is a bear loose in the woods; some people think the bear is vicious, others think it isn't, and some don't even see the bear -- but since there is some question about the bear, isn't it wise to be at least as strong as the bear? The ad is said to end with a silhouette of a man standing up to the bear.

"They are pretty tough," one senior Reagan adviser said.