And now, once again, it's Rosty time on the tax-reform front. The Republican Senate has done its work and reaped bipartisan praise for its version of the massive restructuring of the revenue system. The measure now goes to the House-Senate conference, where Chairman Dan Rostenkowski (D-Ill.) of the House Ways and Means Committee will be the key player, the man with more clout and maybe more at stake than anyone else.

This is his big moment -- the chance to put his personal stamp and his party's political imprint on the most significant overhaul of tax law in a generation. If his aides and colleagues are correct in guessing that Rosty won't stay around the House much longer, this is almost surely the chance of a legislative lifetime for him.

Yet, when I picked my way through a waiting room full of lobbyists to see the chairman last week, I found him in a curiously downbeat mood. His first words were: "I'm concerned about how we approach this conference."

One concern, it developed, was the momentum behind the Senate version of the bill. It's been building ever since Finance Committee Chairman Bob Packwood (R-Ore.) and his key Democratic ally, Sen. Bill Bradley (D-N.J.), recast their legislation around the simple and appealing notion of two low rates -- 15 and 27 percent -- for individuals and a tidy 33 percent for corporations.

President Reagan embraced the Senate bill, editorialists praised it and a huge Senate majority lined up behind it. Rosty would never concede a tinge of jealousy, but he did say, "I want to be sure we've got a level playing field," in which both House and Senate versions get a fair look from the conferees.

Although rates in his bill run up to 38 percent, he said he is not opposed to a 27 percent maximum on individual taxes. "I think we can get 27 percent if we get the reforms in the corporate area we House Democrats put in," Rosty said. "But the Senate pushed the panic button on IRAs Individual Retirement Accounts and sales-tax deductibility, and we have to straighten that out. They can't have it both ways."

When I saw him, he was in the midst of doing what a good Chicago politician always does: ensuring the loyalty of his own troops. He was talking, one by one, with other Ways and Means Committee Democrats, satisfying himself that if he put them on the conference committee with the Senate, he could count on their not going behind his back to cut their own deals with Packwood. The way he put it was, "I'd like to be comfortable that the House members are supportive of the House position."

Rosty is not burdened by philosophy. "I'm a negotiator," he said. "I don't have the pleasant task of saying this is purity." Packwood is much the same. Both are political pragmatists, ready to dicker and deal. And both understand the personal and party stakes.

This tax bill is the big domestic initiative of Reagan's second term and a potential 10-strike for the GOP. Boston pollster Brad Bannon told Democratic leaders in Seattle last weekend that the Senate bill "is very dangerous" to them because it reinforces the idea that Republicans represent change, while Democrats defend the status quo.

Rosty sees the problem. Referring to Reagan's quick embrace of the Senate version of the bill, he says, "I'd be blind if I didn't see they're trying to steal the political credit." When Reagan introduced the legislation, over a year ago, Rosty recalled, "I was told by the House Democratic leadership : Be sure it's a Democratic bill and be sure we get the credit. I did that, when almost no one thought I could. Now the Republicans have passed a completely different bill and they're grabbing credit. I can't do anything about it."

Ah, but he can -- or at least he can try. He will do his damnedest to set up the conference so the price for lower rates and continued middle-class benefits (such as IRAs) must be paid by higher corporate taxes and more loophole-closing. If Republicans let the fight take that shape, then Democrats may salvage some credit for themselves -- and maybe a useful election issue.

As for Rosty's personal stake, those who know him best say he'd love to be speaker when Tip O'Neill steps down later this year, but they doubt he will challenge Majority Leader Jim Wright (D-Texas). And they also doubt he will serve very long in Congress under a Speaker Wright. Some of his key aides are already making plans of their own -- outside Congress.

So this is it for Rosty -- his chance to show he is not just "a palooka" the late Mayor Daley picked 28 years ago to carry his water in Washington, but a heavyweight who can shape a major tax reform and maybe help his hard-pressed party find an issue.

"I think I'm a smart Democrat," he told me. "When a conservative Republican president offers a reform as generous as this one to middle-class and working people, my father would turn over in his grave if I didn't take advantage of that."

Rest assured, Rosty will try to take advantage.