He was interceding on behalf of Israel regarding its National judgment (Lk.21:23), for denying Him as the Messiah and giving Him up for death; Lk.23:18-34 is giving the account of when the people of Israel, along with their leaders where before Pilate when he was determined to let the Lord go, but they "denied the holy and just one, and desired a murder to be granted unto you"(Acts.3:13-14). Saying "let his blood be upon us and our children" (Mt.27:25).
In the verses preceding the Lord's prayer to the Father, He is being led to the cross, and a great company followed and began to cry, but the Lord said to them "daughters of Jerusalem, weep not for me, but weep for yourselves, and for your children. For the days are coming, in which they shall say blessed are they that are barren.... shall begin to say let the mountains fall on us..." (Lk.23.20-31). This is the same language when He spoke of the national judgment of the tribulation, and the day of the Lord's judgments mentioned in Revelation during the opening of the seals (Rev.6:14-17; Mt.24:19-22).
During the gospels, the Kingdom was at hand, but in the early Acts (period) the Kingdom is being offered (due to the
Lord's intercession to forgive them), "And now, brothers, I know that you acted in ignorance ("for they know not what they do"), as also did your rulers. But what God foretold by the mouth of all the prophets, that his Christ would suffer, he thus fulfilled. Repent therefore, and turn back, that your sins may be blotted out, that the times of refreshing may come from the presence of the lord, and that he may send the Christ appointed for you (Christ would have returned if they repented, and restored the Kingdom to Israel), Jesus, whom heaven must receive until the time for restoring all things...here their national forgiveness is being offered, to receive their covenanted kingdom promises (Acts.3:17-21,24-26; cp.1:6). But we see the rejection continue because the leaders of the nation interrupt the offer (Acts.4:1-3).
Christ said during His earthly ministry that to speak a word against him would be forgiven, but blasphemy against the holy spirit would not be forgiven, and then speaks about how they would be persecuted and brought before leaders and synagogues (Lk.12:8-12; cp.23:35). We see this fulfilled in Acts with Stephen who was filled with the holy spirit, which the leaders resisted and then continue to stone him (Acts.6:5,9-12,51,54-60). Now the judgment of the tribulation was set to come, the time of Jacobs trouble (Mt.24.21; Jer.30.7). This is seen in Stephen's vision of the Lord "standing at the right hand of God", whereas in all other places, He is said to be "sitting at the right hand", but not in the book Revelation, where He is said to be standing before He opens the seals of judgment (Rev.5:6). Similar language is also used in the O.T "Arise, O God, judge the earth: for thou shalt inherit all nations" (Ps.82:6).