Two guys from Illinois have given the nation reasons for gratitude this Thanksgiving season. Ronald Reagan and Dan Rostenkowski have met the large-scale challenges of the summit conference and the tax bill, respectively, and come up winners.

Along the way, these fellow-products of what Col. Robert R. McCormick liked to call "Chicago- land" have demonstrated that it is dumb to dismiss the determination of anyone dogged enough to stick with the Cubs through 40 years of unending frustration. Some of us in the press forgot that -- to our embarrassment.

I have been part of a minuscule journalistic minority in believing and writing that the tax overhaul Reagan launched last spring would not be nibbled to death by the lobbyists and lawyers enveloping the House Ways and Means Committee.

It seemed clear to me, early on, that Rostenkowski, the street-smart pol from Chicago's West Side who is the Democratic chairman of Ways and Means, was not going to let the bill die on his doorstep just to please the lobbyists. From the first, he saw far more to be gained by helping the man from Dixon clean up the tax code a bit than by gutting the bill and letting Reagan berate the House Democrats for protecting the special interests.

The bill that Rostenkowski and his committee produced is not, he acknowledged, "a perfect law. Perhaps a faculty of scholars could do a better job. . . . But politics is an imperfect process."

That quintessentially Chicago view of things is right on target, and I think most voters will agree that Rostenkowski has moved the tax code in the direction of fairness. The measure will pass the House next month, I think. And next year, while the Senate may well want to take a look at some of the business tax provisions that could jeopardize America's competitiveness in the world economy, senators will find it hard to derail the Reagan-Rostenkowski propoposal.

A bill that takes 6 million low-income people off the tax roll, cuts individual taxes an average of 8.4 percent, preserves state and local tax deductibility and makes corporations and the wealthy pay their share of taxes despite their sophisticated write-offs is going to be dangerous to sidetrack once it has begun to move.

You can expect the lobbyists to work doubly hard in the Senate, and there are fewer members there who share Rostenkowski's determination to press for action. But I still think the Senate will act. Once people see an attractive option, it is hard to keep them from pursuing it.

That last observation applies, just as much, to the process that President Reagan began last week at his summit meeting with Soviet leader Mikhail Gorbachev. It, too, will be dangerous to abandon, now that it has begun.

Clearly, there were no major substantive agreements in Geneva, and the two sides are as far apart on questions of nuclear arms control and Third World revolutions as ever. Both Reagan and Gorbachev were honest and self-confident enough to admit as much.

But the voters I talked with in Connecticut last weekend -- ranging from hawkish submarine builders in Groton to dovish professors in New London and Middletown -- looked past those disagreements to the main point: The dialogue that began in Geneva has the potential of future substantive progress. It is a lot more attractive option than a continuing exchange of long-distant insults, while Russia and the United States add to their nuclear arsenals.

"If you don't explore what's possible, you never know what might be available," retiree Donald Arthur of Somerville, N.J., said over coffee in Groton. "I don't trust the Russians, but if they get to understand us better, maybe they'll distrust us less. And then, who knows?"

The doubts that were expressed in this column and many other presummit commentaries about divisions in the American government and deficiencies in Reagan's preparations for the meeting, happily proved to be unfounded in Geneva. They may return to plague us another time, when the policy questions of arms control are directly on the table.

But in Geneva, Reagan swept many of them away in his eagerness to engage Gorbachev in direct and extended personal conversation. The respect that apparently developed on both sides, along with their desire to continue talking, changes the atmosphere and may in time improve the chances for substantive agreement.

Neither a new tax law nor a super- power accord on arms control has been achieved. But an important set of procedural barriers has been cleared, and the way is much more open now to deal with the critical issues in their own terms.

The two guys from Illinois have earned our thanks this Thanksgiving for what they have done.