Shaykh ʿUbayd al-Jābiri (رحمه الله) said:[1]
Partisanship (taḥazzub) is defined as aligning with an individual due to his person, regardless of whether he is correct or mistaken. This entails showing loyalty and disloyalty around that person. So whoever behaves as such is described as a partisan (ḥizbī), and he is described as such out of reprimand and disapproval (of his conduct), and so that others who are unaware are made aware of his (condition).
Partisanship is an innovation.
Regarding such a partisan (mutaḥazzib) or anyone else who proceeds upon an innovation, there are two situations according to Ahl al-Sunnah:
The first situation is that he is described with innovation, it is said: “An innovator (mubtadiʿ)”. This is when the revealed proof (al-ḥujjah al-risāliyyah) has been established against him.
There are two ways in which this occurs:
The first is when he opposes a matter that is known to be from the religion by necessity, whether it relates to an obligation or prohibition, and he remains obstinate, refusing to desist, and persisting upon (the error). This (matter of obstinacy is mentioned here) because it may be the case that he has (genuinely) made a slip, and when the (truth) is made clear, he would retract (his view).
The second situation is when he proceeds upon an innovation, and persists upon it even after the proof has been established against him by competent advisors who are skilled in debating in the best way, and who establish the evidence for his error. If he retracts, he remains upon the Sunnah, otherwise, he is an astray innovator (mubtadiʿ ḍāll).
There is another situation: describing someone as being “immersed in innovations” (ghāriq fī al-bidaʿ), or “fallen into innovations” (wāqiʾ fī al-bidaʿ) or “put to trial” (maftūn). These (words and descriptions) are permissible, and they do not necessitate tabdīʿ.
Shaykh ʿUbayd (رحمه الله) has explained here that a person can be called “ḥizbī” when he displays partisanship to an individual, and this is done out of reprimand and disapproval to make others aware. Ḥizbiyyah is an innovation, and just like with innovation in general, even though a person falls into it, he is not declared an innovator (mubtadiʿ) except after the proof is established against him, even though he may be referred to as a “person of innovation” (ṣāhib bidʿah), as Shaykh ʿUbayd has explained elsewhere.[2] This means that saying so-and-so is a ḥizbī, or is part of a ḥizb, does not entail tabdīʿ automatically.
This occurs either in relation to matters known from the religion by necessity, such as certain obligations and prohibitions, and when a person persists with obstinacy, with bigotry to a view or an individual he is following in that view, then he is described as an innovator (mubtadiʿ).
Or it can occur when he proceeds upon innovation (from which is taḥazzub, partisanship) and persists upon it after advice and correction by competent people who advise and establish the proof against him in the matter. So he either retracts or he is an astray innovator (mubtadiʿ ḍāll).
As for using language such as “wallowing in innovation” (such as ḥizbiyyah), or “steeped in innovation”, or “falling into innovation”, and what is similar, this does not necessitate tabdīʿ