When Baltimore Mayor William Donald Schaefer took his road show to Gaithersburg this week to offer technical assistance toward easing traffic congestion on Montgomery County's busiest commuter route, he made only two mistakes: He called Del. Ida Ruben, the well-known head of Montgomery's House delegation, "Dina," and then, compounding the error, "Irene."

On his third try, Schaefer got it correct. He and Ruben, master politicians both, smiled their way through the mayor's gaffe.

Indeed, smiles were the order of the evening when Schaefer came calling late Monday, breezing through unusually light rush-hour traffic on I-270 in his chauffeur-driven city car.

"Montgomery County has helped us on numerous, numerous occasions," Schaefer said in announcing the loan of several city highway engineers to Montgomery to expedite the planned $125 million widening of I-270. "We have a lot in common."

It was an improbable statement from a politician who has tangled so frequently with Montgomery's legislative delegation. Their disagreements have included a bitter wrangle last winter over distribution of state transportation funds.

But for one hour this week, past disputes and those now brewing in the Maryland General Assembly were put aside as Schaefer captivated his audience of about 40 politicos and others with one-liners, observations about city-county relations and one self-deprecating crack about his own political ambitions.

"It was a virtuoso performance -- he touched everybody's nerve endings," said Edmond F. Rovner, an aide and political adviser to Montgomery County Executive Charles W. Gilchrist. Schaefer "wants to be a statewide figure," Rovner said. "Whether that means governor is now anybody's guess."

If Schaefer wants to be elected in 1986, the temporary allies he picked up with his foray into the Yuppie heartland can only help him. After his promise of assistance, several Montgomery officials were fairly bursting with praise for Schaefer, their one-time nemesis. Half a dozen delegates and senators even jostled one another to crowd around the mayor during a television interview after his speech.

Del. Diane Kirchenbauer, the Silver Spring Democrat who helped arrange the meeting with Schaefer, said there was "a real significance in the mayor's coming. After last year, when a big chill developed between us and the city, this trip could usher in a new era of understanding."

Kirchenbauer had more than a passing interest in the mayor's visit. In November, a fund-raising event for Kirchenbauer was hosted by Daryl Plevy, a Silver Spring lawyer and Baltimore City employe who is in the mayor's inner circle of advisers. Kirchenbauer said there was no connection between the fund-raiser and Schaefer's trip to Montgomery.

The genesis of this week's unusual trip was more prosaic, the freshman delegate said. Earlier this month, Kirchenbauer talked with state Transportation Secretary William K. Hellmann (a classmate at Glen Burnie High School during the early 1960s) about Montgomery's transit woes. Hellman, who once headed the city agency where the loaned highway planners now work, arranged for Schaefer to visit the county and offer assistance to Montgomery.

If Schaefer's visit helped to further burnish the mayor's image as a politician willing to set aside personal and geographical differences, it also put in bold relief the political jockeying occurring in Kirchenbauer's district.

Kirchenbauer's political mentor, state Sen. Stewart Bainum Jr., is frequently mentioned by some observers as a running mate for Schaefer in 1986. If Bainum leaves his Senate seat, Kirchenbauer would almost certainly run for it, her colleagues said.

But so could Ida Ruben, another Silver Spring legislator whose legendary tenacity and forceful personality would make her a leading contender for Bainum's Senate seat. Ruben was furious that Kirchenbauer was able to make hay out of the Schaefer visit at the expense of the House delegation leader, several legislators and county officials said.

Still, like the others who met ith Schaefer, Ruben publicly praised the mayor, saying any assistance from the city or elsewhere would be welcome "because Montgomery so desperately needs improvements to I-270."