Milton's tragic poem starts after Samson has been betrayed by Dalila, captured and blinded by the Philistians. Thus anything revealed about Samson's role as a "national leader" will come as background information. Also note, "national leader" is (1) an anachronism and (2) a misconception about Samson's role as God's champion.
First: "Anachronism" means an object, phrase, etc is used wrongly in reference to a time period. For instance, saying Milton went to Burger King for a hamburger for lunch is an anachronism because it connects the wrong things to the wrong time and place. Also, "national" was a new concept (coined 1500-1600) in Milton's time and unknown during Samson's. Thus it is anachronistic to think about what kind of "national leader" Samson was.
Second: It is a misconception of Samson's role and function as one chosen by God to define him as a "national leader." Samson was selected from before his birth to be in an elite religious order and to liberate his people from the Philistians. Thus it is a misconception to think of him in terms of a modern "national leader."
The reason these points are important is that the anachronism and misconception of "national leader" make it difficult to analyze the text to identify sought after textual evidence pertaining to Samson's role: you won't find anything matching "national leader."
Having said this, we can infer a few things about Samson's leadership from the text. In Samson's introductory remarks, he says:
Samson: ... [the] Promise was that I
Should Israel from Philistian yoke deliver;
... let me not rashly call in doubt
Divine Prediction; what if all foretold
Had been fulfill'd but [except] through mine own default [fault/error], (45)
Whom have I to complain of but my self?
Who this high gift of strength committed to me,
... how easily bereft [deprived] me,
Under the Seal of silence [which I] could not keep,
Examination of this text reveals that Samson blames himself ("complain of but my self?") for his failure to liberate "Israel from Philistian" rule ("yoke"). This tells us that as a leader, he let his people down by betraying their trust and failing in his duty. He recognizes in retrospect that God's promise of liberation "foretold / [would] Had been fufilled" except that he grossly erred and broke "the Seal of silence" by telling Dalila what she pleaded to know so she could betray him. This also tells us that, despite his "high gift of strength," he was a morally and spiritually weak leader who could not keep his promise to his God or fulfill his role to his people. Yet, Samson, too late, reclaims some of his spiritual and moral strength when he turns his back on Dalila after she offers him "recompense" to outweigh (she hopes) her betrayal.
Dalila: If aught in my ability may serve
To light'n what thou suffer'st, and appease
Thy mind with what amends is in my power, 
Though late, yet in some part to recompense
My rash but more unfortunate misdeed.
Samson: My love how couldst thou hope [for], [you] who tookst the way
To raise in me inexpiable hate,
Knowing, as needs I must, [I was] by thee betray'd? ( 840 )
In vain thou striv'st to cover shame with shame,
Or by evasions thy crime uncoverst [disguise] more.