I cannot vote for Labour whilst Momentum have such influence on it.
Momentum membership is now over 35,000, and recent elections to Labour's NEC saw posts going to three Momentum members. They now have significant influence in the NEC (though still not as much as the unions), and if they are successful in pushing through policies such as re-selection, can have a radical impact on what the Parliamentary Labour Party looks like. This is a handy summary
Why is Momentum problematic? Many reasons
The then Vice Chair was reported to have made antisemitic comments on a number of occasions, including Holocaust Day. Momentum were slow to react to concerns, and she remains an influential person within the group.
Complaints by women about bullying have been denied and downplayed
Momentum is accused of forcing out various council leaders through sustained campaigns of bullying and harassment. For example, most recently in Haringey, but also in Merseyside.
it has been noted that many influential Momentum members are former members of Militant, the far-left group that Kinnock kicked out of the Labour party in the 1980s.
Momentum members have shown themselves to be very TRA minded - everyone's favourite Women's Officer Even Though I Am Not A Woman is a Momentum member, for example. Whilst JAred O'Mara has been described as "Momentum's first MP", which is clearly demonstrates either a lack of due diligence or (more likely) a lack of concern regarding outright misogyny.
Oh, and they are under investigation by the Electoral Commission regarding their spending in the last General Election
So all in all, I cannot vote Labour whilst Momentum are pulling their strings