In Baltimore, where registered Democrats outnumber Republicans nearly 9 to 1 and most agree that political office is won in the Democratic primary, the hottest issue on the eve of today's election was a proposed amendment to the city's charter that would establish rent control.

A coalition of community, tenant and church groups put the issue on the ballot with a massive petition drive last spring. Since then the group has been fighting court challenges to the proposal and a campaign by opponents armed with a sophisticated direct-mail drive, a San Francisco consulting firm and a $275,000 bankroll.

The charges and countercharges in the bitter feud and the victories and defeats in skirmishes along the way have kept the rent-control issue at the center of attention. It has hardly been noticed that mayor city council and comptroller races are on the ballot, too.

Mayor William Donald Schaefer, a 58-year-old Democrat whose eight years in office have coincided with the city's own urban renewal success story, is seeking a third term.

"Don Schaefer has convinced everybody that he is the city." City Council President Walter S. Orlinsky, a fellow Democrat and the mayor's peskiest critic, has said. "An attack on Schaefer is an attack on Baltimore. Nobody wants to be unpatriotic, so nobody takes him on."

Republican lawyer Samuel Culotta, a 54-year-old former legislator and perennial office-seeker, has challenged Schaefer, but the mayor has virtually ignored him.

"The mayor has not campaigned," said Pete Culotta, the challenger's brother and campaign aide. "He feels he can rest on his laurels. He has refused an opened debate. He thinks everything is fine, but we disagree."

The rent-control debate centers on "Question K," a charter amendment that would roll back rents in Baltimore to their November 1978 levels with some adjustments for utility costs. It would also establish a landlord tenant commission to rule on any future increases.

The proposal's fate on the ballot, according to both sides, will depend on who turns out to vote. Rent control advocates expressed concern that low-income tenants, precisely the voters they need to win are often those who stay away from the polls.

Rent control is needed, according to campaign organizer Richard Gatto, because there is so much demand for apartments that landlords can simply tell their tenants to "pay or get out."

The opponents, however, argue that rent control will lower property tax revenues from rental units and force property taxes up for homeowners, according to spokesman Howard Campbell.

This was the message the rent-control foes, the Keep Baltimore Best Committee, sent out in their most recent mailing to voters. The political literature went out in envelopes stamped with a seal almost identical to the city seal and marked with notice closely resembling a city tax bill.

The rent-control advocates branded it a "hideous" attempt to trick voters into believing Question K would cause a tax increase.

Keep Baltimore Best responded that while the envelope was meant to be "'an attention grabber . . . it was carefully designed so that absolutely anyone who took a look at (it) would immediately be aware that it was not in fact a Baltimore City property tax bill."

Even when the election is over, the rent-control advocates if victorious, still will face a challenge in local court.Opponents filed suit contending the admendment is unconstitional, and the matter was left pending last month.