One of Chicago's smallest, nastiest and most important aldermanic campaigns drew to a close here today with control of the racially divided City Council at stake Tuesday in two runoff ward elections.

One election pits two Puerto Rican candidates against each other in the redrawn 26th Ward, where neither won 50 percent of the vote March 18. Mayor Harold Washington is backing Luis Gutierrez. Alderman Edward R. Vrdolyak, the mayor's arch-rival, supports Manuel Torres.

The race is considered a tossup. Mud-slinging and a vigorous exchange of vote fraud charges in the 26th have eclipsed the race in the redrawn 15th Ward on the South Side. Washington's candidate, Marlene C. Carter, is rated a likely winner over incumbent Alderman Frank J. Brady, a member of the white-dominated council majority.

Led by Vrdolyak, the 29-member majority of the 50-member council has bottled up many of Washington's initiatives and key appointments since Washington was elected in 1983 as the city's first black mayor.

In last month's special elections, ordered after seven Vrdolyak-controlled wards were redrawn to increase minority representation, Vrdolyak retained three seats and Washington picked up two, narrowing the tally at 25 for Vrdolyak to 23 for Washington. Runoff victories by Washington's candidates would yield a 25-to-25 tie -- and the mayor holds the tie-breaking vote.

With Carter, a black, expected to carry the now heavily black 15th Ward, the attention -- and the headlines -- have centered on the bitter fight in the 26th, which includes some of the city's most decrepit neighborhoods.

The contest has a special Chicago-style history. Gutierrez's initial victory at the polls survived several court challenges by Torres. But as Gutierrez prepared to join the mayor's council forces, the Board of Elections, with close ties to Vrdolyak, decided to include write-in ballots in the tally, putting Gutierrez under 50 percent of the total vote.

U.S. Attorney Anton Valukas and Cook County prosecutor Richard M. Daley will send about 200 assistant prosecutors into the two wards to guard against electoral irregularities. But late today Gutierrez and Torres were threatening legal action over some 800 absentee ballots cast by voters organized by Torres' forces.

A televised debate over those ballots turned into a shouting match on a local news show tonight, with Vrdolyak and Washington accusing each other of election fraud.

"Anybody who has a grain of sense in his head would say there's something wrong here," the mayor said. He called on Daley and election Commissioner Michael Lavelle to investigate "each and every ballot."

"He sounds like a loser," retorted Vrdolyak, who then accused Washington of "lying to a judge" to help Gutierrez in last month's election.

"I have no quarrel with everything being looked at," Vrdolyak said.

Whatever the outcome, the losing side is expect to challenge it in court.

Alderman Edward Burke, an ally of Vrdolyak and chairman of the powerful Finance Committee, told reporters last weekend that he doubts the contest for council control will be settled before next year -- when the mayor and aldermen are up for reelection.