In one of the strangest flip-flops in an unpredictable political year, the critical Illinois Republican primary appears to have become a two-man race between Rep. John B. Anderson and Ronald Reagan.

Anderson, until recently the darkest dark horse in the GOP race, has moved ahead of Reagan in polls released today by this city's two major newspapers.

The new odd man out is George Bush, last month's hot candidate. The bottom has dropped out of his candidacy in this large, Midwestern state, according to polls in The Chicago Tribune and The Chicago Sun-Times.

Bush, a former CIA director and ambassador, has plummeted 18 percent since the New Hampshire primary in both polls, taken before his defeats in the South Carolina, Alabama, Florida and Georgia primaries.

Anderson, who represents a Rockford, Ill., district, has picked up the support Bush lost, apparently on the basis of his strong second place finishes in the Vermont and Massachusetts primaries last week. The Tribune poll shows him leading Reagan by 33 to 30 percent; the Sun-Times poll by 39 to 30.

Last month Bush led Reagan in polls by both papers, but his is a weak third in both now.

The results astonished even Anderson. "I don't know whether to believe it," he said yesterday. "It would be beyond anything we had hoped for . . . naturally I am pleased."

A third-place finish in the March 18 primary would be devastating for Bush. Illinois is the first major Midwestern state to hold a primary this year, and it will take the third largest delegation to the Republican National Convention in Detroit.

It has long been a must state for Bush, the place where he had hoped to derail Reagan. "I am emerging as an alternative to Reagan," he told the Chicago Exchange Club on Feb. 29. "I will do well in the South. But I see Illinois as the watershed."

After looking at the polls this morning, Don Pottin, Reagan's Midwestern chairman, said flatly, "I think Bush is dead."

The Bush campaign is understandably frustrated with the dramatic change of events here, and baffled about how to combat it. "We're in a helluva spot in Illinois," said George Kangas, Bush's state chairman.

"We're getting stung by Ford threatening to get in the race. We're getting stung by Reagan's wins in the South," Kangas said. "We're getting stung by questions about whether George can regain his momentum. And we're getting stung by the lack of movement among party leaders in Illinois."

Bush had hoped to pick up endorsements from a series of state and local officials orginally committed to Texas former governor John B. Connally and Sen. Howard H. Baker Jr. (R-Tenn.) when they dropped out of the presidential race. But there has been little such movement.

Bob Blair, Baker's former state chairman, has joined the National Draft Gerald Ford Committee, and is attempting to convince former Baker and Connally supporters to join him. Today he said 37 of the 51 who were running as Baker delegates have agreed to support Ford if they're elected delegates and Ford becomes a candidate.

"I'm interested in winning in November, and I'm convinced Ford is the only Republican who can win," Blair said in an interview. "I don't think John B. Anderson can get the nomination, although he may be able to win in the fall. I think Reagan can win the nomination, but he can't win in the fall."

Although Baker and Connally have withdrawn from the race, their names will remain on the "beauty contest" part of the Illinois ballot. Convention delegates are elected separately and are not bound by the result of the beauty contest.

Dealing with the Anderson phenomenon poses a dilemma for Bush. Until Anderson finished a surprisingly close second to Bush in the Massachusetts primary and to Reagan in the Vermont primary, Anderson was considered merely another one of the also-rans in the GOP field, a candidate too liberal and outspoken to be a legitimate contender for the party's nomination.

Now he may have the potential to eliminate Bush from serious contention.

"Anderson might carry the state and carry it by a lopsided margin," said Bush chairman Kangas. "But what does that do over the long run? John Anderson simply is not a national candidate."

Bush has tried the same tack. In campaign stops in the state Tuesday, he repeatedly said that a vote for Anderson is a wasted vote, and that only Bush can stop a Reagan drive for nomination.

So far, however, the argument has had little impact. Although he is distrusted by much of the party's structure in the state, Anderson's popularity has grown by leaps and bounds, especially among young people, Democrats and independent voters.

Independents and Democrats are allowed to cross party lines and vote in the GOP primary.

Anderson has been endorsed by both Chicago newspapers. The Sun-Times today said he is "a remarkable candidate of courage and integrity, one who refuses to shape his plans and principles to conform to this or that pressure group. Instead, he values the welfare of all Americans."

And an accompanying editorial cartoon pictured the 58-year -- old white-haired congressman shaking hands with Abraham Lincoln, who said, "Good luck, John Anderson."

Bush's best chance to damage Anderson and Reagan will come during a televised debate Thursday night. But polls suggest and some observers feel that the former ambassador has been hurt, not helped, by his performances in earlier debates.