It was the toughest test in years for Roscoe Cunningham's Republicanism, but when it was over, Roscoe had emerged with his reputation for party loyalty intact - more or less.

At Lawrence County Republican Committee's end-of-summer picnic, Cunningham, the loquacious state legislator, was called upon to introduce Dr. Dan Crane, the party's nominee for Congress here in Illinois' 22nd District. Roscoe, of course, had introduced countless GOP candidates at countless GOP picnics, but this was different. For Roscoe also had sought the congressional nomination this year, and had been defeated, in an upset, by Dr. Dan Crane.

It was a big moment, then, when Cunningham came to the podium, and be made the most of it. "I hereby retract all the nasty truths I uttered about this man during the primary," Roscoe said, his rich voice ringing out over the laughter of the crowd.

"I find it easy to support Dr. Crane. After all, I was trained as a lawyer. I can defend almost anything."

It was not exactly the world's warmest endorsement, but for Crane, it was enough. At least he had his whole party behind him, which put him one rung up on Terry Bruce, the Democratic nominee for the congressional seat.

Last week, in a bombshell that rocked the 22nd's Democrats, Don Watson, a veteran party member who had lost to Bruce in the Democratic congressional primary, announced that he could not support Bruce in the November election.

In a whirlwind tour of the district's biggest towns, Watson asked his Democratic friends to vote for the Republican nominee.

Bruce, a popular state senator, was hardly pleased by Watson's defection (particularly because Terry knew it was partly his own fault), but he was not completely surprised, either. Both Crane and Bruce, who are engaged in a tight race for the open House seat from this rural patch of central Illinois, have discovered a sad truth of politics: primary scars are slow to heal.

In theory, losing primary candidates are supposed to rally behind the successful nominee in the general election. This should be especially true in Illinois. Illinois' March 21 primary was the earliest primary in the nation, and the legislature scheduled it then for deliberate reasons.

For one thing, late March is usually a raw, chilling time of year in Chicago, and that is the Democratic machine's favorite kind of weather for an election. The poor climate generally assures a low turnout, which assures victory for the machine's chosen candidates.

But rural legislators, too, like the March primary, because of Illinois history of brising intraparty battles. The early election day is supposed to provide an unusually long cooling-off period before the fall campaigns begin.

When the congressional primaries in the 22nd District were over, a great deal of cooling off was necessary. For the two Republicans whom Crane beat and the three Democrats who ran behind Bruce, defeat was a painful blow to the ego. It helped a little to get mad at somebody. The winner was the obvious target for each loser's wrath.

Crane, who was familiar with that feeling (he had entered, and lost, a congressional primary in Indiana in 1966), took quick steps to offset it. He sent warm personal letters to the men he had beaten, and met with each to praise their conduct during the primaries.

It worked, Crane get a $1,000 contributions from Gene Stunkel, the hardcharging businessman who ran third in the GOP primary. Eves Cunningham, who was shocked when Crane defeated him, eventually agreed to support the Crane candidacy.

But Bruce, who has never lost an election in his eight years in politics, handled his primary win differently. Although Bruce is known as an exceptionally warm and amiable individual, something in him kept him from showing those qualities to his former foes. "I won," he explained brusquely a month after the primary. "They ought to come to me."

Don Watson felt differently. In his office in his hometown of Olney (just two blocks down the street from Bruce's campaign headquarters), he grew steadily angrier as he waited, week after week, for some word from the winner.

One day, about three months after the primary, the mailman brought a letter from the Bruce headquarters. Eagerly, Don ripped it open. Inside he found a letter from Bruce's campaign manager asking registered Democrats to help in the fall campaign.

Don, an easy-going sort, was perturbed by the letter; his wife Jo Ann, a spirited woman with a fiery loyalty to her husband, was impassioned. She had spotted Bruce years ago as an obstacle to Don's political career. This just confirmed her worst impressions.

The more Don and Jo Ann talked it over, the angrier they grew. Late in July, they invited Dan Crane to a steak dinner at the Watson home in Olney. Before the first course was served. Don had offered to endorse and campaign for Crane.

Watson's offer prompted a hearty dispute within the Dan Crane camp. One group of aides said Dan should jump at the offer; if nothing else, the defection of a prominent Democrat would dishearten Bruce's backers, and thus impair his fund-raising. Another faction feared a Watson endorsement might boomerang, generating sympathy - and votes - for Bruce.

In the end, Dan and his wife Judy decided on a middle course: they would have Watson announce his endorsement, but they would take a poll on the reaction to it before they scheduled Don for any regular campaigning.

The Watson-Crane bombshell received front-page coverage in almost all of the 22nd's newspapers. And the campaign made page one the next day, too, when Bruce traversed the district to respond. At the least, Watson had assured that the 22nd's residents would be aware of their congressional campaign.

But it is not just people here who are following the race. For reasons that make sense only in the arcane world of presidential politics, some of the nation's top politicians have their eyes on this obscure campaign for Congress. . .