If you asked any state legislator about Francis Xavier Aloysius Fagan today, chances are good that only a few would remember him.

Fagan--known as "Nutsy" to his friends--was a mythical constituent of Cambridge Rep. Charles F. Flaherty, who wrapped up every legislative session by summarizing it in verse. And while Nutsy fell by the wayside after the 1982 legislative session, Flaherty has remained a permanent fixture in the House.

No longer an amateur versifier, the Cambridge lawmaker is primed to take over the House's top post next year, replacing outgoing House Speaker George Keverian '53 (D-Everett).

But things are going to be a little different on Beacon Hill when Flaherty gets the second most powerful position in the state. Republican William F. Weld '66 will take over as governor in January, along with a new legislature that is markedly more conservative than the "pro-government" government Flahery would like to see.

Despite Flaherty's 24 years of experience in the State House, odds are that the happy days of Nutsy Fagan's Democratic legislature will seem far away. But fellow legislators and state government observers say they think Flaherty has what it takes to assert a progressive agenda while keeping tight control of the House.

"Charlie gets his points across and will get his points across as gently as possible," says Rep. John H. Flood (D-Canton) who is leaving the House next year after an unsuccessful run for governor. "But he won't be afraid to assert himself."

That assertiveness, he says, will be critical to the new speaker's success as he tries to chart a new course for Massachusetts, particularly with the Republicans in control of the executive branch.

"Instead of carrying the administration's policy, because of partisanship, in many cases it will be [the House's] job to oppose it," he says.

But the real question facing Flaherty, says Rep. Peter I. Blute (R-Shrewsbury), is whether he can get a new, independent legislature to go along with him.

Starting next year, Republicans will have enough votes in the Senate to sustain a gubernatorial veto. In the House, they will bring their numbers to 38. In a body of 160, that's still not a lot, but legislators say there is a block of new Democratic representatives who will be swayed by Weld's mandate as governor.

But if anyone can keep the traditional liberal agenda alive on Beacon Hill, observers say, it is Flaherty. A graduate of Boston College High School and Boston College, the majority leader is known in the House for his humor and his persuasiveness.

Legislators say that Flaherty is no Keverian protege. While Keverian came to power in the House in a coup against then-House Speaker Thomas W. McGee (D-Lynn), advocating rules of reform, some legislators have gone so far as to call Flaherty a potential strongarm leader.

But after the past two years, in which many blamed Keverian for the state government's paralysis in dealing with a mounting fiscal crisis, many lawmakers say they look forward to a little discipline on the House floor.

"He's a tough leader," says Rep. Alvin E. Thompson (D-Cambridge) who played high school football with Flaherty's younger brother and has known Flaherty for more than 25 years. "He's not an easy leader."

Flaherty is a persuasive colleague who will not tolerate House Democratic leaders voting against legislation that he wants passed.

"If you're going to be part of his leadership team, you're going to have to have the same agenda," says Thompson.

At a meeting of the Cambridge Democratic Committee before last month's election, Flaherty described himself as "more liberal and less forgiving than Keverian."

City officials say that Flaherty, who has faced only weak opposition since he won his first election in 1966, has been a strong and effective representative for Cambridge.

City Clerk Joseph E. Connarton says Flaherty was instrumental in finding state funds for Cambridge when the antitax measure Proposition 2 1/2 passed in 1980, threatening cities and towns with severe financial difficulties.

"Since that time, he has been a constant supporter of Cambridge," Connarton says.

But Flaherty also knows state government and doesn't go in for rhetoric, Thompson says.

"When you go in to talk to him, with a solution or a problem, you better have your material together because he knows one side of state government to the other," he says.

Flaherty is also extremely easy to get along with, Thompson and other colleagues in the House say. His working relationship with Weld--a fellow Cambridge resident--is expected to be good, they say.

Flood says the first year will see the two working closely together, as the new administration gets to know its way around the State House and tries to solve the fiscal crisis.

"Once things are back on track, you'll see a little more partisanship," says Flood.

Flaherty has also said that he has an effective rapport with his counterpart across the hall, Senate President William M. Bulger (D-Boston), who was reportedly on less than friendly terms with Keverian last year in the toughest days of the state's budget crisis.

Most say they are satisfied that Flaherty isn't the kind of leader to let personal matters interfere with the state's work.

And the increase of partisanship in the House doesn't have to stop the next speaker from getting the job done, says Rep. Robert A. Havern '72 (D-Arlington), who has known Flaherty for more than 20 years.

"Basically, votes always follow a message," Havern says. "And there's nobody who can send a message like Flaherty."