Is it mandatory that we have to support/work for any Islamic party/organization that I think is the best (following the laws of Islam) to form Islamic government/state?
Because in the Quran Allaah Says: "Together hold Allaah's Rope (Islam) and don't get separated from each other." [Aal-Imraan: 103]. If so does it mean that if I pray 5 times a day, fast during the Ramadhaan, follow other laws of Islam that will not be enough for me since I am not following 1 Ayah of the Quran?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

We have already issued Fatwa 88772 on the ruling of joining Islamic parties. You have to know that religion is not restricted to establishing the prayer and fasting only, but religion also includes orders and prohibitions. A person is obliged to do everything that Allaah has ordained and everything that He commanded His Prophet to do, and a he is obliged to refrain from anything that Allaah prohibited, or prohibited His Prophet from doing. Therefore, you should know that every Muslim should abide by the Saying of Allaah (which means): {And hold fast, all of you together, to the Rope of Allaah (i.e. this Quran) and be not divided among yourselves.}[Quran 3:103]. So, if a Muslim does not abide by the above verse, then his prayer and fasting will be of no benefit to him, and will not make him sinless. Moreover, the Saying of Allaah (which means): {And hold fast, all of you together, to the Rope of Allaah (i.e. this Quran) and be not divided among yourselves.}, encompasses more things than what you imagine, and it is interpreted by many matters, the reality of which depends on the Book of Allaah and the Sunnah of the Prophet . It is for this reason that Al-Bukhari entitled a chapter: "The chapter of holding fast to the Book of Allaah and the Sunnah of the Prophet ."

Ibn Hajar said: 'Al-Karmaani said in relation to the above title: "This interpretation is taken from the verse in which Allaah Says (what means): {And hold fast, all of you together, to the Rope of Allaah (i.e. this Quran)…}, because what is meant by the Rope of Allaah is the Book of Allaah and the Sunnah of the Prophet .', in addition to holding fast to Allaah's orders, obeying Him, performing acts sincerely for His Sake.

Allaah Knows best.