The possibility that the White House and Democratic Party leaders have guessed wrong on the pivotal issue of tax reduction is raised by grim reelection prospects for liberal Democratic Sen. Floyd Haskell.

The consensus here is that Haskell has been dropping for five months and either trails Rep. Bill Armstrong, his conservative Republican foe, or soon will. The just-published Denver Post poll shows an 8-point Armstrong lead among probable voters. Raw figures give Haskell a 40-point lead but only 41 percent; less than 50 percent at this stage usually spells defeat for an incumbent.

The national significance: No Republican has geared his campaign to the Kemp-Roth tax-reduction bill more closely than Armstrong. A close friend of Rep. Jack Kemp and an original co-sponsor of his bill, Armstrong wants this election to be a tax referendum.

Haskell is more than agreeable. He is skeptical even of the Senate's $25-billion tax cut and describes Kemp-Roth as "totally irresponsible." His beautifully crafted 30-minute TV program does not fudge; while pointing to big budget deficits, Haskell declares, "We can't at the same time have a tax cut."

Instead, Haskell calls for a balanced budget. That may seem peculiar for a senator whose economy rating by the National Taxpayers Union is a subpar 27 percent (compared with Armstrong's 74 percent). Just last year, Haskell warned that a balanced budget now would produce "one of the damnedest recessions we've ever had."

However exotic for liberal Democrats, the balanced-budget formula is the antidote devised by party strategists to neutralize Kemp-Roth. But one such national strategist, after studying Colorado polls, told us: "It's not working for Haskell." His recommendation: "Hit Armstrong as a right-winger."

There is indeed a vivid contrast. Ex-tax lawyer Haskell, 62, a transplanted Jerseyite and Harvard man who has not lost his cultivated Eastern accent, is viewed by Republicans as typical of leftish newcomers who have transformed Colorado's Democratic Party. Ex-businessman Armstrong, 41, a native Nebraskan, is derided by Democrats for wearing his evangelical Christianity (he was "born again" four years ago) on his sleeve and is considered typical of Republican rightward drift here.

In 1970, Armstrong was Republican leader in the state senate while Haskell was Republican whip in the house. Haskell's unorthodox Republicanism was increasingly challenged by fellow Republicans, and he switched parties the day after Richard Nixon's 1970 invasion of Cambodia. In 1972, tapped as the sacrificial Democratic candidate, he defeated the supposedly impregnable Sen. Gordon Allott. Whereas Colorado politicians a decade ago raced for the middle of the road, Haskell and Armstrong exemplify the ideological polarization of 1978.

Abandonment of the center is underscored by contrasting attitudes on the Occupational Safety and Health Adminstration (OSHA). Haskell has opposed even a small business exclusion. Addressing an appreciative Rotary Club audience in Fort Collins, Armstrong advocated outright abolition of OSHA because "it has done more harm than good."

Polarization is more dramatic on national defense. Armstrong declares defense spending must go up; Haskell pledges "to lower the nuclear buildup," attacks "clearly excessive" weapons programs and promises he will "not let them get away with waste."

Haskell's handlers have no trouble finding outrages in Armstrong's record. Newspaper ads attack his votes against funding for senior citizens, handicapped persons and solar energy. A newly published flyer attacks his votes against the Equal Rights Amendment, Headstart and congressional ethics. "He is a candidate without sensitivity, without a heart," Haskell campaign manager Jean Galloway told us.

Are Coloradans all that concerned with sensitivity and matters of the heart? ?I'm afraid not," a Democratic national committeeman told us. "Our hope is pollution, not good works." With the "brown cloud" hovering over Denver, even Republican politicians worry about Armstrong's environmental voting record, especially his support of the auto industry on emission standards at high altitude.

It is the heart of Armstrong's strategy that votes on auto-emission standards are minor compared with his call for lower taxes. In the first of three non-televised debates, Haskell called Armstrong's proposals "truly, truly dangerous." Off the stump, he compares Armstrong to a Texas demagogue of 40 years ago, W. Lee (Pass the Biscuits, Pappy) O'Daniel, who promised "$40 every Monday" for each head of household, then called for his hill-billy band to play when somebody asked where the money would come from.

But wasn't O'Daniel elected governor and didn't he nose out Lyndon B. Johnson for the Senate in 1942?

"Is that so?" Haskell asked. "I didn't know that."

A few days later, the Denver Post's poll showed times may not have changed.