The current Reformed Theology module has given a chance to read more on the above subject, on which I've posted before, and also before that. Here is a quote from A A Hoekema, tracing the idea that the covenant of grace is a wider category than election back to Calvin himself.
As is well known, certain Reformed theologians have insisted that, strictly speaking, membership in the covenant of grace is identical with membership in the circle of the particular elect— in other words, that only the elect in this particular sense are members of the covenant of grace. This position was held, among others, by Abraham Kuyper in the Netherlands and, more recently, by Herman Hoeksema in this country. It has even been contended by proponents of this view that the conception of the covenant just described was the only one which was genuinely Reformed. I propose to show, however, that this view of the covenant of grace is not the one we find in Calvin.
In Book III of the Institutes, Chapter 21, sections 5 to 7, Calvin makes clear that he does not identify membership in the covenant of grace with particular election. (With the exception of two paragraphs, the material found in these three sections was added in the 1559 edition; it therefore represents Calvin's mature theological thought.) In III, 21, 5 Calvin asserts that God's choice of Abraham and his posterity was an example of his gracious election. He means here, not election in the usual theological sense, as referring to individuals chosen from eternity to be saved, but election in a wider sense, as the choice of a nation to be the recipient of God's special revelation and the object of his special care. In section 6, however, Calvin introduces a distinction into his conception of election:
"We must now add a second, more limited degree [gradus restrictior] of election, or one in which God's more special grace [gratia magis specialis] was evident, that is, when from the same race of Abraham God rejected some but showed that he kept others among his sons by cherishing them in the church."
The "more limited degree of election" must mean what we have previously identified as particular election.
[A A Hoekema, 'The Covenant of Grace in Calvin's Teaching', CTJ 2/2 (1967)]