HAD they all gathered on little Monhegan Island together sometime between 1903 and 1918 to jostle and encourage each other like soccer teammates, who knows what the dazzling outcome might have been. But painting is never collaborative even among insurgent fellows with similar visions. Painters and New York friends Robert Henri, George Bellows, Edward Hopper, Rockwell Kent, Leon Kroll, and Randall Davey drifted separately from coastal Maine to Monhegan and back again in those years. Craggy and lonesome, the coast of Maine touched each artist differently, yet with plenty of immediate impact. Smitten is the old word to describe their collective condition. Blown away is the new. They chattered about the power of what they saw; they wrote letters to each other, critiqued one another's work, and painted the same coves and cliffs. They exchanged tips about paints and the tools of their discipline, all designed to best capture Maine and Monhegan. Most of all they painted the green churning surf, the rolling of blue denim skies, tiny boats against massive black rocks, and the bright light of yellow suns. The result, seen in ''The Allure of the Maine Coast: Robert Henri and His Circle, 1903-1918,'' at the Portland Museum of Art, is a rare exhibition. Brought together for the first time, these 60-plus paintings are a brilliant ensemble and glorious definition of what Maine has done to these gifted men. Henri led the pack. Most of the painters had been students of his in New York before falling to nature's lures. Always the renegade, Henri organized The Eight in New York City to challenge the status quo of the National Academy of Design in 1908. He was also part of the Ashcan School, that cluster of American Realist painters who refuted popular romantic taste and painted urban character from street and curb level. When Henri saw the naturalness of Maine's sea and woods, he was delighted. He was particularly captivated by Monhegan Island, some 17 miles off the coast. ''Maine is not as rugged and strong as I expected,'' he wrote to his parents, and he went on to praise Monhegan Island for dramatic 200-foot-high cliffs facing an open sea. ''This is the real thing,'' he wrote. ''I've never seen anything so fine. From the great cliffs you look down on a mighty surf battering away at rocks - or you can descend and get a side view of the cliffs from lower rocks ....'' On Monhegan, Henri painted with broad, quick brush strokes. In his first visit there he did 25 small oil paintings in four days. ''Marine-Break Over Sunken Rock'' is typical of his use of sea colors. Its vigorous, splashy textures create a seascape that looks to easily done, but when regarded across a series of sea paintings becomes a quintessential vision. But his later career emphasized portraits, and seascapes no longer intrigued him. Henri encouraged the other painters to follow him. Kent, a socialist, saw tiny Monhegan and its few stalwart residents - all fishermen - as a kind of utopia where everyone was equal and everybody worked together. He stayed six months on his first visit, confessing his desire to be like the men there, and ''how I envied them their power to row.'' Kent's ''Down to the Sea'' is an idealist tableau in dark browns and blues of island families gathered against a massive sky to send sturdy men off to sea. In ''The Seiners,'' Kent paints a shaft of sunlight falling in the distance while a dory in the foreground is filled with men straining against oars and dark clouds boil overhead. Both ominous and optimistic, the painting mirrors Kent's ideals and his Monhegan experience. He bought a house on the island and lived there as a handyman and painter for more than a year. When Henri saw a group of Kent's Monhegan paintings in 1910, he observed that the work ''is a proclamation of the rights of man, the dignity of creation. It is [Kent's] belief in God. It is what art should mean.'' Later Henri and Kent argued over which artists to include in a New York exhibition and never resumed their friendship. For Bellows, who painted New York's waterfront and the physical energy of prize fighters, Monhegan apparently helped raise his exuberance level while he became more interested in color and compositional theory. His previous paintings, done elsewhere in Maine, lacked the vitality of the Monhegan work. He wrote to Henri that ''he was working with colors, and not much hue [providing] a lot of new discoveries for me in the process.'' His ''Green Breaker'' indicates that he also moved closer to his subjects. In this painting, filled with luminous green and snowy white, Bellows paints one crashing wave, or at least provides a shortened view of a small cluster of rocks hit by a big wave with another rolling in and cresting to break. After staying for the summer of 1913 on Monhegan and sharing painting forays with Davey and Kroll, Bellows told Henri, ''I have 125 [canvases and panels] all covered with paint and expect to have a few more when I leave.'' Hopper, more moody and introspective than the others, painted Monhegan landscapes and harbor scenes with an emphasis on smooth mass. Cragginess was not a Hopper interest. None of the paintings were at all like his later, haunting paintings with their almost electric loneliness and stillness. This exhibition may be one of the few where the totality of the subject matter is little changed from the summers between 1903 to 1918. A few more houses and trees dot Monhegan now, but it still has not a paved road. And short hikes around the island can put a visitor in the same locations where these painters set up easels and painted what they saw. * 'The Allure of the Maine Coast: Robert Henri and His Circle, 1903-1918' remains at the Portland Museum of Art through Oct. 15.