Once upon a time in American politics, a campaign had some mystery. Those were the days when a frontrunner was hard to handicap and a dark-horse candidate could slip by unnoticed for weeks and still cross the finish line first. Politics was a game with surprise endings.

But the age of the poll has arrived and ruined all the fun. In place of election-day anticipation, we now have "margin for error." Who wants to bother to bet on a race after a "representative sampling" of voters tells us the result weeks or days in the advance of the election?

Polls have turned politics into a slide-rule science, replacing old-fashioned political instincts with quantitative analysis. They fly in the face of the democratic process by scaring off good candidates and telling voters whom they should consider a "serious" candidate.

They also greatly influence newspaper coverage of the campaign with the most attention given to the leading candidates. A good showing in the polls assures a candidate better exposure in the media and makes his job of raising important campaign funds all the easier.

Maryland's gubernatorial primary would be a much different race today if not for the published poll. State Senate President Steny Hoyer may not have dropped his hopes in favor of the lieutenant governor's slot on Acting Gov. Blair Lee III's ticket if not for poor showings in early polls.

State Attorney General Francis (Bill) Burch got out of the race on the last day for withdrawing as a candidate after a newspaper poll showed him losing ground. He was afraid the poor showing would prevent him from raising enough money for the media blitz he considered vital to his chances.

Former State Transportation Secretary Harry B. Hughes and Baltimore City Council President Walter S. Orlinsky are both effective and experienced candidates for governor. But both men have had troubla raising campaign funds and getting media attention because of low standings in the polls.

The candidate who has most benefited from the polls is Baltimore County Executive Theodore C. Venetoulis. Because he has been the only contender who has steadily increased his percentage of the straw vote, he soon became known a the only longshot with a chance of beating Lee.

Last week a poll published by The Batlimore News-American showed Venetoulis trailing Lee by a 3 to 2 edge. But the fact that the Baltimore Countian had narrowed the gap since an earlier poll was seen as a significant boost for Venetoulis as he entered the final days of the campaign.

"Up until now, "explained his campaign manager, Jackie Smelkinson, "we have to prove that Ted can win. People will see that poll, and if they were hesitant up till now, they'll know they can vote for a winner if they want change. If I were a Lee strategist, I would be extremely anxious."

No one is more aware of the psychological impact of a poll than the candidates. To blunt the impact of the poll, Lee forces began saying last week that Venetoulis was a few percentage points away. When the poll finally came out, they signed in relief that he wasn't closer.

Lee has been able to command about a third of the straw vote for months, certainly a subsantial base in a four way race. But his failure to increase his percentage of the voters' poll is a sign to Venetoulis strategists that the Lee campaign is "static" and has already "peaked".

Candidates also take their own polls and try to leak the results if they turn up flattering. Even before the campaign began in earnest last year, Lee leaked a poll showing that he was not identified in the public mind with his predecessor, convicted Gov. Marvin Mandel.

Venetoulis aides are constantly trying to peddle their in-house polls showing their candidate surging ahead in various counties. A recent canvass was taken of the youngest. Democratic voters in each household, an approach that seemed designed to benefit the youthful Venetoulis.

The published poll is relatively new phenomenon in Maryland. Just 12 years ago, the public had little idea that George P. Mahoney, a quirky maverick, would pull off the upset of his generation by defeating two well-known and well-financed candidates in the Democratic primary for governor.

On the day before the election, newspaper stories focused on 11th hour efforts of the two "leading" candidates, State Attorney General Thomas B. Finan and Rep. Carlton R. Sickles. A few obligatory paragraphs were reserved for Mahoney at the end of the stories.

There is little chance of that happening again this year. Newspapers seem to see their polls as insurance policies as well as voter guides. Two days before the Sept. 12 primary. The Baltimore Sun is planning to publish a poll showing the standing of Democratic candidates.

So much for the realm of the unknown.