The Roman Catholic worship of Mary is the most well known, and well disputed aspect of Roman Catholicism. Whereas some Catholics, such as John Paul II, are quite happy to admit Catholicism involves "Marian worship", others claim they simply give Mary the honor (they feel) she deserves. Ultimately, however an individual Roman Catholic approaches Mary, attempts to communicate with her are intrinsic to Catholicism, which even teaches its followers to pray the Marian prayer - a prayer addressed directly to her.
According to Isaiah, those who ask the dead for intercession reveal a lack of light in themselves. In his own, divinely inspired words, "And when they shall say unto you, Seek unto them that have familiar spirits, and unto wizards that peep, and that mutter: should not a people seek unto their God? for the living to the dead? To the law and to the testimony: if they speak not according to this word, it is because there is no light in them."
According to the prophet, and the law to which he referred (Deuteronomy 18:10-12), it is an abomination to seek the intercession of the dead on behalf of the living. God, and God alone, has legitimate claim to our prayers. It is Him we must seek, and His answers we must accept.
King Saul found this out the hard way, when in 1 Samuel 28:9-20, he sought out a witch to summon the prophet Samuel, which she actually did successfully. First, Samuel was rather upset about being disturbed. Without getting too deep into the specifics of where the spirits of the "dead in Christ" (1 Thessalonians 4:16) are, this should be sufficient indication that they want to be left there. Whether soul sleep is true, or they are in some variation of Paradise, they no longer want to concern themselves with the affairs of this life, and should be left alone.
But Samuel had a greater rebuke for Saul: His actions were sinful, and ironically in a way that reverses one of Saul's own good deeds. Scripture, as we have already hinted at, is thoroughly opposed to the living seeking the intercession of the dead. Thus, Saul had actually expelled the mediums and spiritists from the land, a fact that was quite explicitly noted in this very account (1 Samuel 28:3). Samuel's final word on the topic was basically that there was nothing he could do for him. Saul should have sought God, and if God did not answer, that's too bad, because God has the final say. Thus, rather than receiving Samuel's intercession, Saul left the witch with one more sin on his rap sheet.
Roman Catholics have a range of arguments in their defence, including the fact that the dead in Christ are not really dead. As Christ told the Pharisees, "...God is not the God of the dead, but of the living" (Matthew 22:32b). If Abraham, Isaac, and Jacob, are not dead, why must we assume Mary is?
The answer is that, while they are alive in one sense, all of these people are as dead as Samuel in the other sense. God does not desire, and even explicitly forbids, that we should seek their intercession. Instead, we are to seek Him directly, coming boldly before the throne of grace (Hebrews 4:16), for there is but one God, and one mediator between man and God: The man Jesus Christ (1 Timothy 2:5). Prayer to Mary is an abomination, as it is with all the saints. All who practice this sin - called "necromancy" - must immediately repent, being grateful that Christ ever lives to intercede for us (Hebrews 7:25). He, and He alone, is worthy of our prayers. As much as it would be amazing to talk to Mary, we will have to wait until we join her in His Kingdom. At that point, the first thing she will say to all the (now ex) Roman Catholics is "hi, who are you?"