Here is what some of the prominent Muslims aalims have to say about this:
Imam Ibn Qudamah Hanbali
“Stand beside the tomb of the Prophet (s), and say: I have come to you for forgiveness of my sins and to seek your intercession with Allah”
Reference:. Al-Mughni ma' al-Sharh, vol. 3, p. 588; al-Sharh al-Kabir ma' al-Mughni, vol. 3, p. 494.
Imam Ghazzali
“It is recommended the pilgrim should go daily to the Baqi' Cemetery and after saluting the Prophet (s), make pilgrimage to the tombs of (Imam) Hasan ibn 'Ali, (Imam) 'Ali ibn al-Husayn, (Imam) Muhammad ibn 'Ali and (Imam) Ja'far ibn Muhammad (Allah be pleased with them), and also perform the Salat in the Mosque of Fatimah (Allah be pleased with her).”
Reference: Abu Hamid Muhammad al-Ghazzali', Ihya' 'Ulum al-Din, vol. 1, pp. 258-261.
The famous Sunni scholar of India, Shaykh Khalil Ahmad
“Tawassul to the Prophets, saints, pious persons, martyrs and the righteous during supplications, whether in their lifetime or after their death, is permissible. In our opinion and that of our teachers, pilgrimage to the shrine of the Master of Messengers (my soul be sacrificed for him) is the most exalted of proximities, the most important of blessings, and the greatest of means (wasilah) for attaining lofty ranks.
Reference: Shaykh Khalil Ahmad Saharanpuri, 'Aqa'id Ahl al-Sunnah wa al-Jama'ah fi radd al-Wahhabiyyah wa al-Bid'ah, translated into Persian by 'Abd al-Rahman Sarbazi, p. 86.
Opposition
In the opinion of the founder of the Wahhabi sect, Muhammad ibn 'Abd al-Wahhab, and other like-minded 'ulama' of the past, it is permissible to seek help from fellow humans, as during wars and other affairs, if the person or the group who is being asked or entreated has the power and ability to help but asking the deceased for help is not permissionable.
Reference: Nida'-i Wahdat, Ibn 'Abd al-Wahhab's treatise to Shaykh Ja'far Najafi.
Let me also quote this Hadith from the most authentic Islamic book, the Sahi Bukhari.
HADITH
After the Battle of Badr the Prophet (s) stood near the well of the same name and addressed the dead with ayah 46 of Surah al-A'raf. When some of his companions objected that how could the dead hear him, the Prophet (s) replied: "You are not more hearing than them.
Reference: Sahih al-Bukhari, vol. 14, p. 111, Dar Ihya' al-Turath, Beirut; Sahih al-Muslim, chapter 51, pp. 76-77; Musnad Ahmad ibn Hanbal, vol. 1, p. 26 & vol. 2, pp. 31 & 131; Musnad Tiyalisi, hadith 403.