Mayor Thomas M. Menino will announce tomorrow his intention to purchase a building in the North End that formerly housed Mitt Romney’s presidential campaign headquarters as the site for a new school in downtown Boston.

The purchase of 585 Commercial St. would bring the total number of elementary and K-8 schools in the vicinity of the downtown to five, covering a swath of the city that extends through the North End, Chinatown, and the South End.

The building – subject to approval from the Boston City Council and the School Committee – initially would be used to house several grades from the Eliot K-8 School in the North End while a second campus for that school is renovated so it can accommodate more students, city officials said.

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Once the Eliot project is completed, the city would then renovate 585 Commercial St. into a new school for approximately 500 students in kindergarten through grade 8. The new school is expected to open in September 2016.