Boston — Down East Democrats may soon be beating the political bushes for a candidate to challenge Republican US Sen. William S. Cohen next year. While prospects for toppling the GOP incumbent appear dim, opposition party leaders have no intention of letting him have an easy reelection ride.
Gov. Joseph E. Brennan, who is being urged within Democratic circles to oppose Senator Cohen, has indicated he will decide in the next few weeks.
Should his answer be ''no,'' efforts can be expected to persuade former Gov. Kenneth M. Curtis to run. The latter, who was US ambassador to Canada from late 1979 to early 1981 under President Carter, has shown little interest in a try for the Senate or any other elective office. Mr. Curtis, now practicing law in Portland, is coordinating Walter F. Mondale's presidential campaign in Maine.
Although he has not held state office since completing his second four-year term as governor in early 1975 and has not had his name on the ballot since November 1970, Curtis remains well known throughout Maine, his boosters note.
It is generally agreed among state political observers that Governor Brennan would be the stronger Democratic foe for Senator Cohen, who wrested his seat from Democrat William D. Hathaway in 1978 by a wide margin.
Were the governor to run, he would be risking little beyond perhaps his reputation as a winner. Regardless of the election outcome he would be assured of a job, since his gubernatorial term runs through 1986. What might help Mr. Brennan make up his mind to try for the Senate is the Maine constitutional ban on his seeking a third four-year term.
Although declining to speculate what his party might do should the governor decline to run for the Senate next year, Democratic state chairman Barry Hobbins suggests there are several possible candidates. Few, however, may be well known around the state.
Except for the six-year Hathaway term, Mr. Cohen's senatorial seat has been in GOP hands for seven decades. Margaret Chase Smith, its most recent previous Republican occupant, sat there for 24 years.
While there is no shortage of politically ambitious Democrats in Maine, most of them, including Mr. Hobbins, appear to have their electoral sights on other offices, either in 1984 or beyond.
The state Democratic chairman, a sixth-term state representative from Saco, is expected to seek the now GOP-held congressional seat in the First District, which was narrowly won last year by US Rep John R. McKernan Jr.
Democrats now outnumber Republicans by about 23,000 among Maine's nearly 735, 000 registered voters, which include more than 261,000 independents.
Senator Cohen, a Republican moderate, served three terms in the US House from Maine's Second District before moving to the Senate in January 1979. Something of a political maverick, particularly in his early years in Congress, the one-time mayor of Bangor gained national attention in 1974 when, as a member of the House Judiciary Committee then investigating the Watergate cover-up, he spoke out in favor of impeaching Republican President Nixon.