The apostle Paul frequently asked other fellow believers in Jesus to pray for him.
Col 4:3
At the same time, pray for us, too, that God may open a door to us for the word, to speak of the mystery of Christ, for which I am in prison.
Rom 15:30
... join me in the struggle by your prayers to God on my behalf,
James asks presbyter believers to pray over other believers.
Jam 5:14
Is anyone among you sick? He should summon the presbyters of the church, and they should pray over him and anoint (him) with oil in the name of the Lord
Prayer appears to be the normal way for believers (saints) on earth to support one another. Probably all of us have asked another Christian, as Paul did--another saint in the Body of Christ--to pray for us when we had a need.
As Catholic Christians - we believe that as we can ask a fellow Christian -a saint- to pray for us, we should be able to ask prayers from the saints already united to the Lord in heaven.
If the prayers of certain Christians here on earth seem to possess special power because of their great faith witness or holiness, how much more powerful and effective might not the prayers of those of the communion of saints in heaven who are fully united to God. In the book of Psalms, which was the hymn book for the Temple in Jerusalem, we sing to those in the heavenly court and exhort them:
"Bless the Lord, O you his angels, you mighty ones who do his word, hearkening to the voice of his word! Bless the Lord, all his hosts, his ministers that do his will!" (Psalm 103:20-21, RSV, as below)
The fact that those in the heavenly court can hear our prayers is also indicated in the book of Revelation, where we read:
"And another angel came and stood at the altar with a golden censer; and he was given much incense to mingle with the prayers of all the saints upon the golden altar before the throne; and the smoke of the incense rose with the prayers of the saints from the hand of the angel before God." (Revelation 8:3-4)
Thus those saints who are angels have a role in presenting our prayers to God in an intercessory manner. (Angels are also saints, as indicated by the fact that the Bible applies the Hebrew word for saint/holy one -- qaddiysh -- to them, cf. Daniel 4:13, 23, 8:13. Thus we speak of St. Michael the Archangel, St. Gabriel, St. Raphael, etc.).
Since the Ascension of Christ, when Jesus took the Old Testament saints from sheol to heaven, large numbers of humans saints have also been in heaven, and Revelation indicates they also present our prayers to God:
"And when he [the Lamb] had taken the scroll, the four living creatures and the twenty-four elders fell down before the Lamb, each holding a harp, and with golden bowls full of incense, which are the prayers of the saints" (Revelation 5:
.
The twenty-four elders represent the hierarchy of the people of God in heaven (just as the four living creatures represent the hierarchy of the angels of God in heaven), and here they are shown presenting our prayers to God under the symbol of incense (which is, in fact, what incense symbolizes in church, since it is a pleasing smell which rises upward).
One might object, saying, "But maybe those weren't prayers to the saints but prayers to God!" This may well be true. However, a person who says this only digs the hole deeper for himself since this would mean that those in heaven are aware of prayers which weren't even directed to them!
In any event, we know that the saints in heaven (whether human saints or angel saints) are aware of our prayers and, based on them, intercede with God on our behalf. Scripture, Tradition, and the Magisterium all agree.