State Sen. Francis X. McCann (D-Cambridge) and State Rep. Levin H. Campbell III '48 (R-Cambridge) played the third act of their protracted campaign debate before a large and attentive Harvard Square audience last night.

Although the combatants exchanged some discreet accusations of dirty pool, their encounter contained few surprises for those familiar with earlier installments in the forensic series.

Campbell, who is contesting the Senate seat McCann has held for ten years, began by observing that he had "tried to conduct my campaign on a high level" and did not care "what my opponent and his supporters do."

All the same, he told a group of 120 in the basement of the First Unitarian Church, there was "one thing that has troubled me but has not particularly surprised me." That, he explained, was some unsigned campaign literature mailed out last week in Somerville, quoting him "as saying things I didn't say."

McCann counterattacked with gusto. "My opponent started on the high-level campaigner routine," he said. "Before the night is over, I will give him the name of one of his supporters who took it upon himself to be a private investigator and dig into my private life."

Once these preliminaries were over, Campbell and McCann reiterated their positions on issues dividing them, notably the three underpasses that are scheduled to be built at the intersection of Memorial Drive with Western Ave, and Boylston and River Streets.

McCann sponsored the act that directed the Metropolitan District Commission to build the underpasses. Campbell has promised to introduce a bill to repeal the legislation if he is elected.

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"From the point of view of Cambridge," Campbell charged, it is "senseless" to build the underpasses. "There is no screaming, crying demand for these things," he told a questioner.

With irony that was appreciated by many in the crowd, McCann said, "I wish--I sincerely wish--I could stand here and take credit for the underpasses." But he could not, he added, because a professor had suggested building over-passes in 1946.