Another Labour MP, Joan Ryan, has tonight announced she is leaving the party to join the Independent Group. This is significant, and not just because it creates a sense of momentum. Ryan is the first Labourite to leave who wasn't involved in the months of secret planning meetings. She was, until fairly recently, arguing that the best thing to do would be to stay and fight to change the party back. Now, she has gone, releasing a potent statement about her reasons.
Those reasons include what she calls 'the scourge of anti-Jewish racism', which 'simply did not exist in the party before his election as leader'. She argues that she has 'fought for as long as I can within the Labour Party'. Ryan is the parliamentary chair of Labour Friends of Israel, and the organisation has already said that she will remain in that role. It would have been difficult for anyone in her position to hear Luciana Berger's description of the anti-Semitism within the party and argue that staying was a good idea, unless they had evidence that Berger's assessment was wrong.
And this is another reason why Ryan's departure is significant. There are other MPs who have been very vocal on the way the Labour Party has approached anti-Semitism, chief among them Ian Austin, who has been on resignation watch for a while. His views on Europe are somewhat different to the rest of the group, though, which makes the move less easy. Then there's Margaret Hodge, who was threatened with disciplinary action after engaging in a furious row with Jeremy Corbyn in the Commons.
Other MPs such as Ruth Smeeth are passionate advocates of staying in the party and changing it. But then so was Joan Ryan, who has now argued that there is just no reason to do so.
This isn't the final move, either. Rumours are rife that a number of Tory resignations are planned in the next few days. At that point, the new group will need to start articulating what it stands for other than leaving the existing main parties.