A controversial government bill designed to reduce widespread tax evasion by the self-employed is pitting Italy's more than 4 million artisans, merchants, small-business owners and professionals against its 20 million salaried employes.

Because of the passion it has aroused, primarily among artisans and shopkeepers, the bill, written by Finance Minister Bruno Visentini, a Republican, also has been threatening the cohesion of Italy's five-party socialist-led government.

The bill, which seeks to enforce tax payments by the self-employed, was agreed to in principle last February by the coalition of socialists, Christian Democrats, liberals, Social Democrats and Republicans in exchange for important labor cost concessions by Italian workers, who, like all salaried employes here, are taxed at the source.

But the anger expressed by shopkeepers and artisans, now preparing their third nationwide lockout in six weeks to protest the bill's new accounting, computation and enforcement clauses, has given pause to Christian Democrat and Social Democrat politicians fearing reprisal at the polls in next spring's local elections. The unions already have staged one four-hour general strike to show the importance they give to the fairness question.

After a marathon session last week, the Visentini bill was forced, more or less intact, through the senate -- but only because the government called a series of six votes of confidence to avoid more than 600 amendments presented by the opposition and to limit desertion from among government ranks.

But the measure, which could bring about $5.4 billion more into government coffers, still must get through the larger chamber of deputies. In the lower house, critics can be expected to make another stab at whittling away some of the bill's most controversial clauses, primarily a strict limitation on the present system of income-splitting in family businesses and so-called "inductive" methods of auditing that will be based on internal revenue service calculations of assumed profits resulting from estimated costs and revenue.

The merchants, spearheaded by Confcommercio, the powerful Italian General Confederation of Commerce and Tourism, say they fear corruption by unscrupulous tax collectors as well as vastly heavier taxes that could force many to go out of business.

Visentini, who has threatened to resign if the bill fails to pass in its present form, says the new law is designed to increase tax collection, not revenue. "I can't understand the fuss," he said last week. "In my view, those who are screaming must be tax evaders."

The tax evasion problem stems from a growing awareness here that the tax burden in Italy is very unevenly distributed. According to statistics released by the unions, Italy's salaried employes earn 55 percent of the nation's gross domestic product but pay 75 percent of the total personal income tax collected.

The union figures, which were not challenged by the government, said that of $152 billion in income tax collected by the government in the decade ending in 1983, $108 billion came from Italy's 14.5 million salaried workers and only $44 billion from other incomes. The study calculated that tax evasion by the self-employed amounted to $40 billion.

Last year, there was widespread outrage and embarrassment here when the government released figures for 1981 showing that the country's merchants, small-business owners and self-employed professionals claimed to have earned on average less than Italy's blue-collar workers.

Opponents of the new bill, such as the Confcommercio merchants association, claim statistics purporting to show such widespread tax evasion by shopkeepers, artisans and professionals have been blown out of proportion. They say the complex Italian tax structure is slowly suffocating many of this country's thousands of family-run enterprises.

In fact, an underlying problem is the low profitability of many of Italy's 900,000 small retail distribution outlets, about twice the number per capita as those in West Germany.

The new law sets up complex tax tables that make the payment of income tax and Italy's heavily evaded value-added, or turnover, tax interdependent.

It allows businesses with an annual turnover of less than $432,000 to choose between the country's complex accountancy system or a simplified method that calculates business expense deductions on the basis of pre-calculated percentages. But it provides for "inductive" methods of tax controls based on calculations about the kind of costs and profits that a certain type of business, in a certain location and with a certain number of employes, ought to have. This latter point is one of the most controversial.

Another key provision sharply revises an earlier system that permitted "splitting," the equal division of income among family members involved in a family business. As originally written, the new law would have forced the proprietor to declare two-thirds of the business' income as his own and allow other family members -- children, parents, spouses or brothers and sisters only -- who effectively participate in management to divide the remaining income for their own tax declarations.

Intense lobbying by the Christian Democrats and Social Democrats forced the minister of finance to alter those proportions to 51 percent and 49 percent.