Most of this is original with me. I am simply putting it all together.
1. Longfellow is at once (a) most popular poet of his own day. (b) One of the most representative / accomplished poet-translators of this same period. (c) 2nd Harvard Smith chair, after Ticknor, and one of the most accomplished Hispanists of the day. Being a triple threat in this way makes him a very considerable force.
2. Unlike Whitman, Dickinson, (or Poe, Emerson, Melville in their own fashion) he was not a precursor of modernism, and was not even a high-brow in his own day. Not until Irmscher's book Longfellow Redux in 2006 was the condescension against him broken. Yet what many would have seen as derivative in his own day and afterwards now seems like a productive, metapoetic intertextuality. Andrew Higgins talks about his engagement with Weltliteratur and Goethean ideas.
3. A lot of his translation from the Spanish is medieval or very early renaissance. Berceo, Manrique, and romancero / cancionero poetry.
4. Another relevant context is Hispanophilia in England (Southey). William Cullen Bryant did some translation in the US, and there are versions by Byron, Walter Scott, etc... of medieval balladry, mostly. Longfellow's anthologies...
5. The historical memory of this is largely lost. Langston Hughes, WCW, Bly, etc... don't refer to earlier traditions of verse translation from the Spanish. Nor do I mention it in Apocryphal Lorca. Despite this, there is continuity, especially in translations of balladry in the civil war period.
6. Discuss continuity between medieval earnest moralism of Manrique and Longfellow's Victorian earnest moralism. The "Psalm to Life" is supposedly influenced the the Coplas of Manrique. In case you don't know this poem it contains lines like "Dust thou art to dust returneth / was not spoken of the soul."