The phrase "an overwhelming portion of" is correct and usable in written English.
It can be used to describe a significant or large part of something, often in contexts discussing opinions, resources, or data.
Example: "An overwhelming portion of the survey respondents indicated that they preferred online shopping over traditional retail."
Alternatives: "a significant majority of" or "a large share of".
If iTunes becomes a dominant radio force, it could control an overwhelming portion of the music business.
It's not an overwhelming portion of this $27 billion market, though — only about $3 billion, without public schools.
Despite the overthrow of President Hosni Mubarak, the military has retained an overwhelming portion of its historic authority.
When such an overwhelming portion of the economic benefits are skewed toward a tiny portion of the population — as has happened in the U.S. over the past few decades — it's impossible for the society as a whole not to suffer.
Former President Jimmy Carter weighed in with Brian Williams of NBC News on Tuesday: "I think an overwhelming portion of the intensely demonstrated animosity toward President Barack Obama is based on the fact that he is a black man".
"I think an overwhelming portion of the intensely demonstrated animosity toward President Barack Obama is based on the fact that he is a black man, that he's African-American," Carter said.
Flavorful shrimp come in a fine, mildly spicy sauce of ground peanuts and plantains, while an almost overwhelming portion of roast suckling pig includes ribs, shoulder leg and plenty of crisp skin.
Most Turks strongly oppose the idea of a religiously oriented government, and the overwhelming portion of Mr. Gul's constituency voted for his party because they said it had done well running the country, not because its leaders were pious men.
He also wrote the screenplays for three of his shows -- "Fiddler," "Enter Laughing" and the 1949 Broadway flop "Mrs. Gibbons Boyss" -- when they were made into films and did a handful of other writing projects for television, but the overwhelming portion of his career was the stage.
Though all the provincial and municipal general hospitals, most of the county hospitals and community healthcare centres, and a small percentage of rural clinics were included as surveillance sites, the overwhelming portion of the rural clinics were not included.
