News that spread through congressional cloakrooms 24 hours before Wednesday's tax showdown not only ended Democratic hopes of finally beating Ronald Reagan on the House floor but also killed the party's stubbornly held notion that voters hate lower taxes.

"I can't believe it," one prominent Democratis congressman told as when he heard the news Tuesday afternoon: Rep. Norman Dicks of Washington state, no "boll weevil" conservative but a regular western Democrat, was defecting to support the Reagan tax package. That opened a hemorrhage of Democrats jumping party lines that finally reached 48 in Wednesday's historic vote.

It also signaled the folly of years of unremitting Democratic opposition to Kemp-Roth tax rate reduction, before and after the bill's embrace by President Reagan. Dicks and the other Democratic defectors were moved by an unprecedented public demand, spurred by Reagan's video virtuosity but reflecting genuine popularity for deep, continuing tax reduction.

The House tax vote capped the passage of Reagan's entire economic package in 200 days, a feat unmatched since Franklin Roosevelt's 100 days in 1933. It also was a unique personal triumph for Rep. Jack Kemp, who began his tax crusade seven years ago against bipartisan ridicule. But in addition, it revealed profound political misconceptions by the Democrats.

They had talked themselves into a series of comforting notions: that Kemp-Roth was a political albatross around Reagan's neck, sure to defeat him for the presidency; that Reagan did not really like Kemp-Roth and never would push it; that across-the-board, multiple-year tax reduction was not acceptable to scores of Republican congressmen, much less to enough Democrats to pass it in the House.

That framework of misconceptions guided Rep. Dan Rostenkowski's strategy as Ways and Means Committee chairman in drafting a Democratic alternative. While appealing to all manner of business special interest groups, he played the old Democratic game of scourging the righ by concentrating tax cuts below the $50,000 income level.

But congressional Republicans knew there was one sweetener Rostenkowski never could match: permanent tax indexing, to adjust for inflation, after three years of tax cuts (originally part of the Kemp-Roth bill but removed by cautious Reagan administration policy-makers). Rep. Barber Conable, senior Republican on Ways and Means, turned the tide a week before the vote when he finally talked the administration into tax indexing.

That had a profound impression on scores of congressmen -- including Norm Dicks, a longtime aide to liberal Sen. Warren Magnuson before his election to Congress in 1976 and certainly no conservative (rated 5 percent by the American Conservative Union last session). Over the weekend of July 24-25, he first entertained thoughts of backing the Reagan bill, thoughts enhanced by a Monday telephone call from Reagan himself.

The president's Monday night speech generated a telephone campaign impressing Dicks, among other congressmen. On Tuesday morning, he had his first personal meeting with Ronald Reagan, in the Oval Office. On Tuesday afternoon, word of Dicks' detection reached stunned Democratic leaders. They knew then that the battle was over.

Rep. Elliott Levitas of Georgia, a southern liberal who had been a Kemp-Roth original co-sponsor but backed away from it in the face of Democratic hostility, contacted Kemp Wednesday. He and Rep. Dan Glickman of Kansas, another former Kemp-Roth Democrat, would vote for the Reagan bill if Kemp on the floor would commend their desire to combat inflation through a third-year tax cut trigger rejected by Reagan. The deal was made. When Levitas announced his support for the bill, a one-sided Reagan win was obvious.

While Dicks, Levitas and Glickman were reflecting the popular appeal of tax reduction, Rostenkowski's votes that had been rounded up through special interest favors were coming unstuck. Two Texans he had counted on because of oil tax concessions, Reps. Ralph Hall and Jack Hightower, voted with Reagan -- an agitated Hightower casting his vote at the last minute after failing to get Rostenkowski's release from his commitment. The chairman's threats that hell would freeze over before he agrees to oil concessions in the Senate-House conference could not surmount Reagan's popular support in Texas.

The $50,000 question" posed by Rostenkowski proved a poor match for yreagan's semi-permanent tax reduction. Nevertheless, in his gracious speech to the House Wednesday, Rostenkowski pledged to campaign against the righ through steeper graduation of taxes "as long as I'm chairman."

In his considerably-less-than-gracious speech closing Wednesday's dcebate, Speaker Thomas P. O'Neill showed he had learned nothing. Beginning by calling this "a great day for the aristocracy," he claimed the nation's big corporations had artificially stimulated that flow of telephone calls to congressional offices. To the very end, Tip O'Neill could not believe that the people really prefer lower taxes to bigger government.