It's actually very simple. Earle has to provide direct evidence of two issues. First that the $200,000 or so that was transferred from TRMPC to the RNC with instructions as to who should be supported in the same $190,000 (I have seen the first figure both exact ($190,000) and approximate (around $200,000) is the same $190,000 sent out shortly thereafter (again (I have seen differing timeframes) to the same specified candidates. Parenthetically, I still find it hard to believe that there were actual written instructions on which candidates it was to go to, which seems both incredibly clumsy and extremely incriminating, but I have not seen that rebutted yet, so I am presuming the incredible is actually true. And that's not a leak - it was part of the public indictments.
That would normally be a very hard obstacle to mount, but given the note, it seems to have been met. The specific challenge with respect to Delay is to show that he as an individual had specific knowledge of that particular laundering. Unless there is another note, which is hard to imagine, the logical speculation, and it is only that, is that a donor recounted a conversation with delay in which he explained what would occur in response to the donor saying that he could not contribute on behalf of a corporation to a Texas campaign. The other reasonable speculation is that another indictee has proffered and testified before the GJ that Delay directed it, but that usually gets out, if nothing else by reporters that watch who goes into the courthouse - targets don't testify, so it would be noteworthy if one was seen entering and leaving the courthouse. Perhaps the GJ was not being actively monitored, but presuming it was, that would have been noted. If so, the former is the more likely scenario, unless the prosecutor truly has no direct evidence..