(The) The main stay.
“Sober ...practical men ... constitute the moral backbone of the country.” —W Booth: In Darkest England (Part i. 2, p. 17).
“A union man, and a nationalist to the backbone.” —T. Roosevelt: T. H. Benton, chap. v. p. 113.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894