The fact that Catholics pray to Mary and other saints is one of the things that fundamentalist Protestants call "un-Biblical" and frequently also assert is a result of "pagan influence" on the Church. But, like all Catholic practices rooted in Church doctrine and dogma, intercessory prayer is Scriptural and was present in the Church since apostolic times.
The first misunderstanding many Protestants seem to make is missing the crucial different between prayer as worship and prayer as mere communication. Worship, of course, is for God alone. But when a Catholic asks a saint to petition God on his behalf, he is hardly worshipping that saint - he is merely making a request, which, in fact, is identical to what Protestants do when they ask friends and family to pray for them.
Another critical misunderstanding - or objection - is the assertion that Christ's status as Mediator between God and man makes intercessory prayer wrong. But this is a mixing of apples and oranges (which is made evident by the Bible's own examples of intercessory prayer, among other things).
1 Timothy 2:5 says "For there is one God. There is also one mediator between God and the human race, Christ Jesus". But in this very book, this very chapter, in fact in the verses just before this verse Paul commands Christians to engage in intercessory prayer!
"First of all, then, I ask that supplications, prayers, petitions, and thanksgivings be offered for everyone, for kings and for all in authority, that we may lead a quiet and tranquil life in all devotion and dignity. This is good and pleasing to God our savior, who wills everyone to be saved and to come to knowledge of the truth." (1 Timothy 2:1-4)
Paul requests that Christians pray for one another. To acknowledge this and yet claim that it is then wrong to ask another Christian to pray for us is nonsensical.
So, now the fundamentalist Protestant will object that it is good and proper to petition Christians still living for prayers, but not those who have gone on to their eternal reward. Scripture also flatly contradicts this view. What is forbidden is the practice of necromancy - divination of spirits for information, earthy reward, etc. That is condemned by Scripture and the Catholic Church. But does Scripture teach that necromancy is the same thing as departed Christians praying for those still on earth? Of course it is not.
Moses and Elijah appear with Christ to the apostles during the Transfiguration. This is proof alone that the saints in heaven can have knowledge of activities and people on earth and are concerned with them.
And the book of Revelations offers a picture of the heavenly saints offering the prayers of the faithful to God on our behalf:
"Then I saw standing in the midst of the throne and the four living creatures and the elders, a Lamb that seemed to have been slain. He had seven horns and seven eyes; these are the (seven) spirits of God sent out into the whole world. He came and received the scroll from the right hand of the one who sat on the throne. When he took it, the four living creatures and the twenty-four elders fell down before the Lamb. Each of the elders held a harp and gold bowls filled with incense, which are the prayers of the holy ones." (Rev 5:6-8).
“Another angel came and stood at the altar, holding a gold censer. He was given a great quantity of incense to offer, along with the prayers of all the holy ones, on the gold altar that was before the throne. The smoke of the incense along with the prayers of the holy ones went up before God from the hand of the angel.” (Rev 8:3-4)
How beautiful are these passages depicting Heavenly intercession of the saints on our behalf? And there can be no doubt about what they depict: angels and other saints offering prayers to God. Since those in Heaven are perfect and have every desire fulfilled, these supplications must come from the suffering Church in exile on earth.
Why is it worthwhile to ask the saints in paradise to pray with us? James declares "The prayer of a righteous man has great power in its effects." It's as simple as that.
[Some of the fundamentalist objections to the holy practice of intercessory prayer reveal a theology so childish and naive it is difficult not to laugh. For instance, Loraine Boettner, a well-known anti-Catholic author, complains that there would be no way for those in Heaven to hear more than a single prayer at once, or to understand supplications in languages they never spoke! This type of thinking reveals of the dangers of building a theological system with no understanding of philosophy, metaphysics, or the vast body of knowledge contained by the 20 centuries of the historical Christian Church. Heaven exists in eternity, outside of time; it does not make sense to believe that the departed are still bound by physical time. Nor does it make sense to think that their minds are still bound by earthly limitations.]
In the end, those who object to the Christian practice of intercessory prayer almost invariably do not even practice what they preach, for they will ask their friends and relatives to pray for them (because they intrinsically realize this is good and proper). The entire Protestant objection to the practice relies on the assumption that those in Heaven can have no knowledge of those on earth, something that has no basis whatsoever in Scripture or Tradition and in fact is directly contradicted by Scripture - and frankly is simply the result of prejudice against Catholicism. It is also flatly contradicted by the documented experience of 20 centuries of Christian practice.