On Friday, Gov. James Thompson summoned the power elite of the Illinois Republican Party into secret session to consider the looming reality of life with Ronald Reagan.

The would-be power-brokers had come to think of tomorrow's Illinois primary results, once regarded as crucial to the Republican presidential nomination, as largely irrelevant. Even if their fellow Illinoisan, Rep. John Anderson, defeated Reagan in the "beauty contest," as signaled by polls, the party leaders saw little prospect that anybody could defeat Reagan for the presidential nomination.

Then Big Jim Thompson, who last month came within a hair of endorsing the now-fading George Bush, gave this advice at the Friday luncheon: keep cool, don't attempt any stop-Reagan drive (as desired by some present) -- especially an eleventh-hour Gerald Ford candidacy, then still a possibility. That means any impetus to keep the nomination from Reagan will not come from Illinois, which once seemed its most probable source.

Actually, the legislative leaders asked to lunch by Thompson -- important county chairmen and other party officials -- still consider Reagan the Republican least likely to beat Jimmy Carter. But they are practical politicians accommodating to reality, as symbolized by a rendezvous between Thompson and Reagan less than 24 hours before the luncheon.

The party elite had sought an early alternative to Reagan by getting aboard John B. Connally's bandwagon last year. Besides doubting Reagan's electability, they wanted to stop Reagan's hard-charging state-chairman, state Rep. Don Totten, from taking over the Illinois delegation to the Detroit convention.

Connally's collapse and Bush's boomlet after winning the Iowa caucuses led Thompson closer to an outright endorsement of Bush than was generally realized. Even after Bush's shattering defeat in New Hampshire, Thompson was still contemplating active support for him.

Bush's last hope for that died with his dismal showing in southern primaries, followed by polls that put him well behind Anderson and Reagan in Illinois. That left two potential alternatives to Reagan: Anderson and Ford.

The Illinois power-brokers, pragmatic beyond any ideology, are not greatly concerned by Anderson's liberalism but are convinced he cannot be nominated by a Republican national convention. That conviction was reinforced at Thursday night's nationally televised debate here when Anderson, proclaiming his intention to build "a new politics," refused to pledge Republican loyalty. h

As for Ford, Thompson and most of his allies suspected it was too late for him. A case in point is former Rep. Tim Sheehan, Jerry Ford's pal from congressional days and a respected party leader here. He twice urged the former president to run without success, then filed as a pro-Reagan delegate. He was asked Thursday if he would switch back to his old friend if Ford entered the campaign. "I'm afraid it's too late," Sheehan told us. He was more prescient that he knew. Within 48 hours, Ford had finally and irrevocably taken himself out of the race.

"Make no mistake," one party leader confided Friday. "We're getting together to stop Reagan." But he was not speaking for Thompson, who had more modest ambitions: electing a majority of uncommitted delegates Tuesday against Totten's Reagan-committed delegates.

Highly important in Illinois, that would mean Big Jim instead of Don Totten would be leading the delegation in Detroit. Those uncommitted delegates as realistic politicians would presumably have little interest in Anderson. They could back Bush if he miraculously revives. But most guess they may end up voting for Reagan, whose nomination many have long dreaded.

Accommodation with reality was in the air when Totten, Thompson's antagonist in Springfield's legislative wars, telephoned to say Reagan would like to see the governor alone. Previously, Totten had prevailed on Reagan to decline Thompson's invitation to an executive mansion dinner of the kind accorded other presidential candidates. A few hours before the televised debate, Thompson, unnoticed, entered Reagan's suite on the ninth floor of the Drake-Hotel (with Totten not present). By all accounts, it was a cordial, forgettable chat with Thompson giving Reagan some tips on energy policy.

That might make it slightly easier to accept life with Reagan. So might Reagan's virtuoso performance in the debate. But on primary eve, Illinois Republican leaders note Reagan's difficulty in expanding his 30 percent hard core in the polls and wonder whether that portends disaster in November. Any movement to make sure that does not happen will have to begin someplace other than Illinois.