There comes a time in every candidate's life when he or she has to choose between principle and politics. Dr. Dan Crane ran heading into that unhappy dilemma last Friday.

More precisely, it was on Friday that Crane, the Reaganite dentist who is the Republican candidate for Congress in Illinois' 22nd District, finally had to deal with the dilemma. But he had seen the conflict coming week ago, and had agonized over it since then.

Crane initially had come down four-square on the side of principle. But when his advisers pointed out the strategic risks implicit in the principled course. Crane's resolve began to falter. Friday afternoon, when it came down to the crunch, practical politics prevailed over principle.

The occasion for the painful and sometimes bitter dispute within the Crane camp was a debate scheduled for Friday night between Crane and his Democratic opponent, Terry Bruce. The debate was to be telvised over WILL-TV, which reaches about two-thirds of the 22nd, a sprawling, rural chunk of eastcentral Illinois.

Crane had been delighted last summer when WILL had offered to broadcast debates between the 22nd candidates. An intense advocate of conservative ideology, Crane saw his campaign more as a "bully pulpit" for spreading his views than as a means to victory.

"I'm not going to compromise my principles," Crane said last winter, when the campaign was young. "If I can't convince the voters I'm right, at least I can say I tried." In his speeches around the district, Crane loved to quote Benjamin Franklin: "To sin by silence when they should protest makes cowards of men."

Crane's campaign strategists, Bill Mencarow and Cliff Downen, had no fears about their candidate's strident protests against big spending and creeping liberalism in Washington. The campaign staff was confident that Crane was winning supporters among the 22nd's conservative electorate each time he spoke.

That confidence began to wane Aug. 11 when Crane and Bruce met in the WILL studio for their first televised debate. Well before the confrontation ended, Mencarow and Downen knew that Crane had been seriously outclassed.

While Bruce offered reasoned, cautious answers, Crane turned the debate into an aggressive harangue against communism.

When Crane described communism as a worldwide monolith, Bruce noted calmly that there were noticeable diffrences among European, Soviet and Asian communists. When Crane was asked about Africa, he launched into an attack on Marxism and digressed to take slaps at actress Jane Fonda and Sen. George McGovern (D-S.D.). "I think the situation in Africa is more complicated that you might just have heard," Bruce responded.

But it was the style of Crane's performance that most disturbed Mencarow, his media specialist. A student of McLuhanism, Mencarow knew that Crane's tendency to grit his teeth and barge ahead emotionally was too "hot" for a "cool" medium such as television. It might fascinate some viewers, but it would frighten even more.

There was not way, Mencarow and Downen decided, that they would let their candidate face his opponent on television a second time.

The candidate was outraged. After all, Crane said, WILL had scheduled a second debate for Oct. 20 and he was not the type to chicken out, and no campaign manager would tell him otherwise.

For weeks the unresolved dilemma hung heavy over Crane headquarters, but events this fall strengthened the position of those who favored backing out.

Conservatives around the country had sent Crane so much money that he could afford and advertising campaign that would provide more exposure than another debate would. Crane's privare poll showed him narrowly leading Bruce; it is standard political wisdom that front-runners do not benefit from debates.

Early this month, though, word leaked out that Crane was waffling. The 22nd's newspapers and a field day - particularly with Crane's contention that he could not fit the second debates into his campaign schedule.

In a biting editorial headlined "Don't Believe Crane Ploy," the Decatur Herald, the district's most widely read paper, observed that "if candidate want to appear on television, they'll find the time. . . . Crane's refusal . . . involves strategy more than time."

This reaction stirred the Crane camp to third thoughts, and the candidate sat down to talk it over again with his wife, Judy. Deep down, Crane knew he should debate.His wife demurred. "What I said," she recalled later, "was the Bill and Cliff know what they're doing so we'd better do what they say."

Thus when Bruce sat down at the debate table in WILL's studio Friday night, he found an empty chair to his right. It was a golden opportunity.

After taking countless potshots at Crane's "confusion" and "simplest solutions," Bruce ended on a tone reflecting anger mixed with sorrow.

"I'm sorry my opponent didn't show up tonight," he said to sympathetic murmurs from the moderator. "I guess he was afraid of what might happen. If he cannot stand up here and debate me, I wonder how he can stand up for the people of the 22nd District in Congress."

Back at Crane's campaign headquarters, workers cringed at Bruce's sally. They cringed again the next morning when they learned that, partly because he had ducked the debate, Crane had lost the endorsement of his hometown paper. Despite their big campaign treasury and their candidate's standing in the polls, Crane's managers were worried.

But those fears were tempered by the knowledge that soon the Crane campaign was going to unleash one of the most potent forces in American politics today.