CHICAGO -- Suburban Republican precinct committeemen have signalled their leaders that they will do nothing to help the campaign for governor by Secretary of State James Edgar, a sign that the long decline of the Illinois GOP is approaching still lower depths.

Edgar has managed to get himself on the high tax side while tax revolt roars through Illinois. The reaction, from not only party workers but ordinary voters as well, has erased a once comfortable lead over his Democratic foe, State Attorney General Neil Hartigan.

If Democrats win the governorship for the first time since 1972, a wipeout of the state Republican ticket from top to bottom looms in November, with commensurate disasters in Cook County (Chicago).

The lesson here is not just that tax increases are as massively unpopular with the public as they are succulent for the establishment. Beyond that, Republicans who appear to voters little different from Democrats have trouble winning elections.

That doesn't square with the spectacular career of departing four-term Gov. James Thompson, a high-tax, big-government, pro-labor Republican. But Big Jim was blessed with extraordinary campaigning skills and spectacular good luck.

Moreover, during his 14 years in Springfield, GOP losses mounted in Congress and the state legislature. The one area of Republican growth -- Cook County, where whites were fleeing domination of the Democrats -- stopped last year with the election of Richard Daley the younger as mayor of Chicago.

Still, Republicans expected to hold the governorship. Edgar, whose official duty distributing license plates gives him statewide identification, was running 20 percentage points ahead of Hartigan. Even Hartigan's lifelong buddies in the closely knit Chicago Democratic organization had given him up for dead.

Confident of victory, Edgar endorsed Thompson's massive surtax. But Jim Edgar could not duplicate Jim Thompson in mobilizing the entire establishment -- from big business to big labor -- behind him. Many captains of industry prefer congenial Chicagoan Hartigan to cool downstater Edgar. One chief executive officer who long has been active in the GOP was dissuaded by subordinates from openly endorsing Hartigan but will vote for him anyway. When Thompson's allies in the labor movement proposed a dual endorsement, Hartigan insisted that if the AFL-CIO did not support him exclusively, forget it. He won the endorsement that used to go to Thompson.

Simultaneously, grass-roots Republicans sick of Thompsonism turned on Edgar. But even when a 31-year-old political operative named Steve Baer ran surprisingly well in the primary against Edgar on an anti-tax platform (after President Bush broke precedent to come here to campaign for Edgar), there was no sign of worry. Republican township committeemen still did not get phone calls returned from Edgar headquarters. After all, wasn't he trotting to a double-digit victory?

He was -- until in late summer Hartigan unleashed a barrage of television commercials on a single issue: taxes. "When given a chance," said the opening spot, "Jim Edgar's first response has always been higher taxes. . . . He voted 13 times for higher state and property taxes. Now he wants the largest tax increase in Illinois history." That was followed by a second generation of commercials: "Jim Edgar and Jim Thompson: 14 years, 15 taxes. And now Jim Edgar wants more time and more taxes."

With the polls suddenly showing dead heat, Edgar retaliated with spots about Hartigan receiving "insider" loans as director of a failed savings and loan 21 years ago. The momentum, by bipartisan agreement, is with Hartigan.

Edgar is in a predicament no Republican ever should face. Addressing a business group at O'Hare Airport last week minutes after Hartigan again attacked the tax increase, the Republican candidate sounded a little like Fritz Mondale when protesting that he at least tells the taxpayers what's in store for them: "I don't want anybody who votes for me to be surprised after Election Day."

Conservative township committeemen are suddenly getting their calls returned, and the Edgar campaign has turned to that old tax-cutter -- Secretary of Housing Jack Kemp -- to come here for breakfast and plead Edgar's cause to invited right-wingers. But that does not quite do the job against a Democratic opponent who has seized the anti-tax ground.

Edgar's slip affects the entire Illinois Republican ticket. Coincidentally, Republican Rep. Lynn Martin, who had planned her uphill challenge against Democratic Sen. Paul Simon by attacking him on taxes, has been undercut by President Bush's breaking of his tax vow. Repudiation of the supply-side Reagan revolution may cost Republicans dearly in November.