The cloying hands of power and politics can work their way even into the deepest wilderness.

Six months ago, Dave Halsey had a dream - to cross Canada the way it ought to be crossed, by foot, pack toboggan, snowshoe, skis and canoe. He would make it from coast to coast in 18 months with a party of four.

Today, Dave Halsey is in Ft. Langley, British Columbia, 50 miles into his trip and all alone. He has parted ways with his party after a blowup over leadership and rights to the spoils of their voyage.

Halsey intends to carry on alone. "My frst thought was that som may people had helped me out, I couldn't let them down."

Last winter, Halsey, 20, quit his job painting houses in McLean and set to work drawing up a 75-page prospectus for his journey. His enthusiasm was fueled by memories of canoeing trips in the Canadian woods as a teen-ager.

In a startling display of perseverence, he sold his plan. The National Geographic Society was first to come forward with aid in mapping the route and a pledge of support, in return for a possible story at the end.

Then Little-Brown, the book publishers, took an interest. A contract for a book is being drawn now, and Halsey has an agent to monitor developments.

Britches of Georgetown offered materials. Halsey's old college fraternity kicked in some money.

The foursome was selected and late last month Halsey ended his dizzying round of negotiations and set off for Seattle to meet his mates. It started badly and got worse.

"They saw it as more of a lark, I guess," said Halsey last week from Ft. langley. His backers had insisted that his mates sign contracts reserving rights to stories from the trek to Halsey. they didn't want to sign.

Then in Vancouver, fears set in. "People were telling us, 'Listen, you can't sleep out in a tent in Manitoba in the winter,'" Halsey said. "That's ridiculous, but the other guys started having doubts about our competency and experience for this kind of trip. They were questioning my leadership ability."

It got worse when the hiking began. Two of the men were falling behind. There were complaints. "I told them 'look, we've got objectives here, we've got a job to do.' It was a real showdown."

The showdown ended with Mark Jusko, 21, of Chicago; Neil Cogen, 24, of Bowie, and charles Bratz, 24, of Stevens Point, Wis., dropping out.

And now Halsey is alone, with no doubts about where he's going.

"I'm taking a crash course in photography so I can still get pictures for the Geographic. They said they'll keep sending film. And I talked to some of the good backwoods people here. They said, 'Hell, it's no big dear, go it alone.'

"Personally, I think it will be much more rewarding. I'll have to do more backpacking and less canoeing and I might change my route some, but I'm going on."

Halsey said the Canadians "just won't let me stop. They're wild about it." The Canadian Broadcasting Corp is monitoring the voyage with radio reports every two weeks and ABC-TV is planning a filming session along the route mext month.

But most of the voyage will be hard and unadorned. "What I'm finding right now is it's just like hiking in Virginia. There are nice folks all along the way."

This is, of course, spring in British Columbia. It will not be so mellow in December in Gypsumville, Manitoba or in February in Pickle Lake, Ontario.

Halsey's route is 4,600 miles. He's got 4,500 to go.