When Dwight W. Mize was 19 years old, he decided to leave the American Indian reservation in Oklahoma where he was born. His mother gave him $20 and he hitchhiked to California to make his fortune.

Today Mize, now 49, is a wealthy real estate developer who has built hundreds of moderate-income homes in California and elsewhere. Yesterday he announced plans to purchase the McLean Gardens apartment complex in Northwest Washington for $30 million and converting it to condominiums.

His impoverished childhood, nevertheless, apparently has left an imprint on Mize's current business outlook. "Being in a minority and starting with little," he said in an interview here yesterday, "I still have a feeling for people who need two incomes to buy a house and for the value of home ownership."

His 4-year-old firm, The Mize Co., specializes in homes that people of modest means can afford. According to Howard Adler, a California developer who is a friend of Mize, most of Mize's frame and stucco houses now sell for $35,000 to $55,000.

"He has a reputation for building a very good single-family home in that price range," Adler said.

The Mize Co. has been cashing in recently on what Californians describe as a housing boom in the Riverside and San Bernardino areas of southern California. The family business - in which all three of Mize's children work - has also built housing in Las Vegas, Colorado Springs and Dallas.

Mize, who has a relaxed and outgoing manner, has combined his real estate career with active participation in Democratic politics. In 1968, Mize was the overall coordinator, friends say, of former Rep. Richard T. Hanna's successful reelection campaign.

Hanna, a California Democrat, was sentenced in April to serve 6 to 30 months in prison for the role in a Korean conspiracy to buy influence in Congress. Friends say Mize ended his association with Hanna long before the events for which Hanna was prosecuted began.

In 1970, Mize ran for the California State Senate in a heavily Republican district in Orange County. He lost by a more than 2-to-1 margin. Friends say he is still a participant in and contributor to Democratic campaigns and holds what some Californians term a political job, a part-time, appointed position as a California inheritance tax referee.

Mize, who was born into the Osage and Quapaw Indian tribes, arrived in California in 1949. His first job was in the electronics industry. But within a few years he had obtained real estates sales and broker's licenses. The builder said he worked at real estate about 60 hours a week and, in addition, held part-time jobs at a factory and a gas station.

During the 1960s and early 1970s, Mize had a series of management positions in large real estate development firms before starting his own company in 1974. In recognition of his American Indian heritage, he has termed his housing projects the "First American Indian Series" and named home models for Indian tribes - Navajo for three-bedroom homes or Osage for four-bedroom models.

He lives in Riverside with his wife Joanne.