I don't think many think Corbyn himself is anti-semitic - although it's been worryingly close to being able to seriously level that charge at him. No, the accusation that has reasonably been laid at his door is that his hatred of Israel has allowed anti-semitism to thrive in the party and that he's been unable to separate the two successfully. His attitude has acted as an enabler for those who hate Jews in the rank and file.
When he challenged the IHRA definition of anti-semitism is a case in point - it was totally unnecessary to do it, but he couldn't help himself.
Because Corbyn hasn't led effectively on the issue, that community (not all but a sizeable amount) are now worried it's the tip of the iceberg, and that Corbyn in power would enthuse these anti-semites even further.
Bercow is basically saying exactly that - the problem exists, but Corbyn himself is not an anti-semite.
This is what “everyone knows”, but it really doesn’t stand up to much scrutiny.
AS or people who stray / appear to stray over the line have always existed on the Labour fringes; Williamson was an MP before Corbyn became leader, Pete Wilsman was on the NEC during Blair’s time, even the things that Corbyn was accused of doing all happened before he was leader. Ken Livingstone was Labour mayor when he compared a Jewish journalist to a concentration camp guard, and Galloway was a Labour MP for nearly 20 years.
Nor was it ever just a Labour issue - the Tories were (are) home to anti Semitic bigots - not only with ancient things like the reaction to Disraeli, but also the pro -Hitler fringe of the 1930s, the open racism of the 80’s and as seen by how they referred to Letwin and Bercow recently. Even Jo Swinson’s Lib Dems had David Ward as an MP for a long time too (and FWIW he is still in the party as a local councillor).
So this is not a new development; it is a long standing issue in British politics and not something that has just started once Corbyn has won.
