Don H. (Del) Clausen, running for his 11th term as the Republican congressman in California's far northwest, had enough troubles already.

His district is only 33 percent Republican. Some towns tied to the forest industry have unemployment rates rivaling those of the Great Depression. His Democratic opponent, a young, energetic state assemblyman, has exploited the fact that a Republican president tried to cut a federal program sacred in the district -- special payments for residents put out of work by the expansion of Redwood National Park.

As if that weren't enough, Clausen is threatened by nuclear war.

"I have bad news for you," said letters sent to 3,000 voters in Clausen's district. "Your congressman has already announced that he doesn't care what you think about the arms race."

The anti-nuclear movement has singled out Clausen for his Aug. 5 House vote for a Reagan administration alternative to an immediate freeze on nuclear weapons stockpiling. The movement's political action committee, PeacePAC, put him on their "Doomsday Dozen" list of congressmen deserving defeat. "Anything that is an organized activist effort obviously has to be taken seriously," said Clausen. "I know this is an emotional issue and it is a matter of carrying out a campaign of education."

In Sonoma County, a farm and forest area now bursting with affluent newcomers escaping from the crowded San Francisco Bay area, the nuclear freeze issue has gained intimidating force. The hills west of Santa Rosa are full of marijuana-growing, politically conscious residents with a strong commitment to disarmament.

The polls show widespread popularity in the district for a mutual, verifiable freeze of Soviet and U.S. nuclear arms. So, the news stories announcing Clausen's vote "against" the freeze -- a distortion of what actually happened in Congress -- did him little good.

The congressmen also did not help himself with a newsletter headlined, "Clausen Reaffirms Commitment to Nuclear Freeze." Doug Bosco, his opponent, has begun to wave a copy of the federally funded mailing at political gatherings.

"The very day that this newsletter was mailed from Washington Congressman Clausen voted against the nuclear freeze," Bosco shouted to students sunning themselves on the grass and concrete courtyard of Santa Rosa Junior College.

The student body president, 18-year-old economics major Lori Fisk, said she favored Bosco, 36, over Clausen, 59. "Bosco in my mind is in favor of the nuclear freeze and I can see by Clausen's personal record that he is not," she said.

Clausen handles the freeze issue as if it were fissionable material itself. In remarks printed in the Congressional Record during the House debate, he labeled as "the product of genuine concern" both a resolution for an immediate freeze, pushed by movement activists, and a resolution for a freeze after reductions to equalize U.S. and Soviet forces, backed by the Reagan administration.

But freezing at current levels, he added, would allow the Soviets to retain their current advantage and remove their incentive to make concessions. So he supported the administration alternative, he said, as a better way "to achieve peace and stability throughout the world."

Clausen, campaigning here, expressed willingness to support other kinds of nuclear freeze proposals, including an immediate freeze resolution on the November California ballot.

But Bosco has broadened his attack to cover Clausen's overall support for increased military spending. "In Eureka," Bosco told a political science class here, "they are saying, 'Hey, we want a job, we want unemployment benefits before we go off into all of this military technology.' "

Two groups, Californians for a Bilateral Nuclear Weapons Freeze and a newly organized "Freeze PAC" lobby, have squabbled recently over control the freeze movement here and in other key districts. The first group, which put the freeze initiative on the ballot, broke with Freeze PAC out of fears that the new group's outspoken liberal slant might alienate Republican supporters of the freeze. Clausen hopes to benefit from that kind of liberal taint to what has been an issue popular even with many California Republicans.

Clausen aides point out that Bosco missed voting on a freeze resolution that passed the state legislature. Bosco responds that he supported the measure, but had to be in Washington to discuss a waste management plan with federal officials.