"How is this different from the Republicans' support of Leiberman? Aren't they trying to split the Democratic vote?"
Correct me if I am wrong, but I don't believe the RNC has given money to the Liberman campaign or sent out mass mailings!!
You may be right about the paper trail of money but it has been clear for some time that Republicans as a group support Lieberman in order to defeat Lamont. I believe Pataki has funneled money to Lieberman as well as others and I'm not at that certain such funnelling would be happening without the blessing of the RNC, outright or tacit.
And if they did send out campaign literature, they would be required to disclose that they sent it. This is written and sent out by the DSCC without any disclosure that it is the DemocRATS sending it. In fact they are trying to make it look as if a Republican organization is sending it.