It had such consideration from the 19th century, with the advent of Romanticism, to the 20th century.
Note that El Poema de Mío Cid wasn't even printed before 1779, as you can read in the book La filologia romanza by Pietro G. Beltrami (in Italian):
Solo nel tardo Settecento e nell'Ottocento sono tornati nel circolo della cultura
testi fondamentali della letteratura medievale di Francia
e Spagna. In Italia, invece, i classici del Trecento sono sempre stati letti,
studiati e imitati, ma questa è una caratteristica particolare della storia
letteraria e linguistica italiana.
Il Poema del Cid, il maggiore testo epico castigliano
delle origini (cfr. n. 277), è rimasto confinato in ambito locale a Bivar
(oggi Vivar del Cid) e a Burgos, nell'unico ms. antico noto,
di cui fece una copia Juan Ruiz de
Ulibarri nel 1596 (entrambi i mss. sono oggi alla Biblioteca Nacional de
España, Madrid, mss. Vitr/7/ 17 e V.a. Mss/6328). Citato solo, e raramente,
in opere erudite, soltanto nel 1779 è stato stampato da Thomas Antonio Sánchez
nel primo volume della sua Colección de poesías castellanas
anteriores al siglo XV. Nei quattro volumi (1779-1790), Sánchez stampa
il Cid, le opere di Berceo (cfr. n. 298), il Poema de Alexandro Magno (cioè
il Libro de Alexandre, cfr. n. 297) e il Libro de buen amor (cfr. n. 330), con
ampi glossari che spiegano parole e locuzioni della lingua antica, gettando
le basi degli studi moderni di letteratura spagnola medievale.
My translation:
Only in the late eighteenth and nineteenth centuries fundamental texts of medieval
literature of France and Spain returned to the circle of culture. In Italy, however,
the classics of the fourteenth century have always been read, studied and imitated,
but this is a particular feature of the story of Italian literature and linguistics.
The Poema del Cid, the major Castilian epic text of its origins
(see no. 277), remained confined to the local area in Bivar
(today Vivar del Cid) and in Burgos, in the only known ancient manuscript,
of which Juan Ruiz de Ulibarri made a copy in 1596 (both manuscripts are now
in the Biblioteca Nacional de España, Madrid, manuscripts Vitr/7/ 17 and V.a. Mss/6328).
Mentioned only, and rarely, in scholarly works, it was only printed by
Thomas Antonio Sánchez in 1779 in the first volume of his Colección de poesías castellanas
anteriores al siglo XV. In the four volumes (1779-1790), Sánchez prints
the Cid, the works of Berceo (see no. 298), the Poema de Alexandro Magno (i.e.
the Libro de Alexandre, cf. no. 297) and the Libro de buen amor (cf. no. 330), with
extensive glossaries explaining words and phrases of the ancient language, throwing
the basics of modern studies of medieval Spanish literature.
In the chapter entitled "Evolución histórica" (that is, "Historical evolution") of the book Las historias literarias españolas: repertorio bibliográfico (1754-1936) by Fermín de los Reyes Gómez (in Spanish), you can read how Spanish medieval texts were
completely ignored in Spanish literarature histories, in the chapter devoted to the eighteenth century:
En paralelo se reeditan obras de escritores del siglo XVI y principios del
XVII (Garcilaso, Lope), en unos años de vital importancia para el desarrollo
de la historiografía. Aún no se tendrá en cuenta la literatura medieval
y, así, durante décadas no tuvieron consideración de textos canónicos
los que hoy son piezas clave como el Poema del Mío Cid o La Celestina.
Tendrá que llegar el Romanticismo para recuperar, de forma más imaginativa
que documental, según expresión de Pozuelo Yvancos, la Edad Media.
My translation:
In parallel, works by writers of the sixteenth and the beginning of the seventeenth centuries
(Garcilaso, Lope), in years of vital importance for development
of historiography. But medieval literature will not yet be taken into account
and, thus, for decades they had no consideration of canonical texts
those that today are key pieces such as Poema del Mío Cid or La Celestina.
Romanticism will have to come to recover, in a way more imaginative
than documentary, in the words of Pozuelo Yvancos, the Middle Ages.
In the book Literatura y nación, you can find an article by Antonio Martín Ezpeleta, entitled "El concepto de nación en la historiografía literaria española"
(that is, "The concept of nation in Spanish literary historiography") that explains how the cliché of national epic poem was developed:
Por su parte, la definición de la poesía o poema nacional también suele
ser un tópico repetido de historia literaria en historia literaria
desde las primeras hasta las del siglo XX, momento donde la labor de Menéndez Pidal
particularmente subrayó más si cabe su importancia para la nación. El
Poema de Mío Cid, como poema épico nacional por excelencia, se convierte
en una suerte de referente para los españoles. Las cualidades del personaje
histórico y literario son trasvasadas a la españolidad en un curioso
proceso mitificador que también alcanza, como veremos, al Quijote. [...]
Lo que significa el Cid
para la poesía épica es lo que representan los romances para la poesía lírica, a los que las historias literarias del XIX cada
vez dedican más atención.
My translation:
For its part, the definition of national poetry or national poem
is usually also a cliché repeated from literature history to literature history
from the first ones to those of the twentieth century, when particularly
the work of Menéndez Pidal
further underlined its importance to the nation. The
Poema de Mío Cid, as a national epic poem par excellence, becomes
a kind of reference for the Spaniards.
The qualities of the character, both historical and literary, are transferred to
the Spanishness in a curious
mythifying process that also reaches, as we will see, Don Quixote. [...]
What the Cid signifies
for epic poetry is analogous to what the
romances represent for lyrical poetry, to which the literature histories of the nineteenth century
pay more and more attention.
It's curious to see in the text of this lecture of 1868, entitled "La poesía épico-nacional en la edad media", how these ideas are expressed. For instance:
Terminaba mi última Conferencia encareciendo la verdad de esta tesis:
—El canto de Gesta, seguido del Poema del Cid, terminan y cierran la épica heróico-popular de
nuestra pátria, representando la unidad nacional, última
forma y complemento necesario de la inspiracion de independencia y nacionalidad
que expresa la épica Asturiana
y la de Castilla.— España sucede por lo tanto á Castilla,
como Castilla había sucedido á Leon y Astúrias. Hoy me
incumbe la grata tarea de demostrar que así es, y de contestar a la crítica francesa,
que se atreve á poner en duda
la originalidad nacional, de esta creacion épica de la musa
popular española.
This is quite difficult to translate for me, but I'll try:
I ended my last lecture by stressing the truth of this thesis:
—The cantar de gesta, followed by the
Poema del Cid, ends and closes the heroic-popular epic of our country, representing the national unity, the last
form and necessary complement of the inspiration of independence and nationality
that expresses the Asturian epic
and that of Castile. — Spain therefore succeeds Castile, as Castile had succeeded León and Asturias. Today I have the pleasant task of demonstrating that this is the case, and of answering the French critics, who dare to question the national originality, of this epic creation of the Spanish popular muse.
As an evidence of the fact that this poem has been completely ignored for centuries, it is interesting to notice how the "cid" entry in the Tesoro de la lengua castellana o española, the first dictionary of the Spanish language, compiled by Sebastián de Covarrubias in 1611, mentions that the name of "cid" is given to Rodrigo Díaz de Vivar in the book of Tales of Count Lucanor, but there isn't any mention to the Poem of the Cid.
