Well the hearing is still ongoing, and there is a lot that the Senators will see that we do not.
So far as I have heard so far, Trump has flirted dangerously with a line but probably has not quite crossed it. The most damning part is the account of a meeting where Comey met Trump alone, and in particular where Trump ordered everyone (including the Attorney General) out of the Office before that meeting. Everyone involved has made clear that this seriously violates proper procedure, and is wholly inappropriate.