While Rep. Thomas P. "Tip" O'Neill Jr. (D-Mass.) promised a nationwide Democratic "tidal wave" at the Fantasia in Cambridge, Independent Saundra Graham's supporters gathered across from the Jack-in-the-Box in Central Square to applaud her victory against incumbent John J. Toomey.

Graham's win came in her second attempt to unseat Toomey in the race for State Representative in the Fourth Middlesex District.

Graham's aides last night predicted that the final count would show a 3-to-2 win for the black city councilwoman, and on "strong evidence" from important wards gave their candidate 6600 votes to Toomey's 4600.

Graham's stunning victory over the 22-year incumbent Democrat came in a backwash of liberal and left-of-liberal votes from various precincts in the district which stretches from Harvard's backdoor along Broadway and Mass Ave.--including Leverett Towers--to the banks of the Charles near M.I.T.

The district includes the largely black Riverside and Cambridgeport areas.

Graham last night attributed her success to the high number of new voters registered in Cambridge this year and the unexpectedly high turnout of people "who normally don't get out and vote-- young people, blacks, and young professionals."

More than 80 per cent of the registered voters in Cambridge turned out at the polls this year. Precincts in which only 500 voters cast their ballot in the 1974 representative race showed increases of from 100 to 200 voters. More than 12,600 people voted in the district in all, up from 8512 in 1974.

Included in Cambridge voters are more than 130 Leverett Towers residents who were recently included in the fourth district after a post-primary decision by the Cambridge Election Commission redrew the boundary lines to include that dormitory and a house on Ames St. in the fourth district.

The Election Commission decision drew attacks last week from Toomey and city councilors who were concerned, they said, that possible "criminal activity" may have taken place to influence the commission.

No specific charges were made, but Mayor Alfred E. Vellucci and four others asked City Manager James L. Sullivan to seek extra observers to watch the disputed precinct's polls