In this line we pray to mary, asking her to pray for us, but for many this causes an issue because they believe that prayer is due only to God. This belief actually comes from one of the definitions of the word pray which is:

“to address God or a god with adoration, confession, supplication, or thanksgiving” (Merriam-Webster)

But this is not the only use of the word, and in fact, it's not even the most common use of the word! The more common use is:

“1: to make a request in a humble manner” (Merriam-Webster)

This definition is especially seen by the middle english phrase “pray tell” which in essence means “please tell me”. when we, as Catholics “pray” to the saints, what we are actually doing is humbly asking them to pray for us to the Lord our God. 

A common objection to this is 1 timothy 2:5 which says that there is but one mediator between God and man (paraphrase) but this is directly refuted even within scripture when st. paul asks for prayers ( 2 thessalonians 3; romans 15:30-32; colossians 4:2-4 and others). When we ask for prayers from the saints, what we are doing is essentially the same as asking a friend for prayers. And specifically, by asking Mary to pray for us we recall the wedding feast of Cana, in which Mary interceded for the wedding guests, and we also should note that Jesus listened. He did as his mother asked. By asking Mary to pray for us, we ask her to bring our needs before her divine son in heaven. 

Some will say, however, that Mary is dead and thus cannot intercede, but this is a denial of scripture, for Jesus reminds us of the words of his father, who said, "I am the God of the living, not of the dead" (Mark 12:27) and indeed he says that those in heaven have "eternal life"! So, not dead. 

But can they pray for us? Yes, yes they can! 

The biggest proof for the intercession of saints actually comes from the book of Revelations , which shows us heaven and tells us that the angels indeed do pray in heaven! For this, here are a list of scriptures:

Prayers of praise to God:

Rev 4:8 

Rev 5:9-10

Especially: Rev 5:11-14

Rev 7:12

Rev 11:15-18

Prayers of intercession:

Rev 6:10

Rev 8:1-4 (these prayers also are shown to have effect in rev 8:5-6, showing the prayers as being heard)

I will say also, for any Catholics, if you look in your old testament apocrypha, in the book of Tobit, Raphael describes himself as an angel who presents the prayers of men before the Lord. All of this goes to show that the “hail Mary” is indeed a completely scriptural prayer, and in no way denotes any worship of Mary. In fact, as we should note from this that more than 50% of this prayer is actually a direct quotation of scripture!