Broad generalization. There will always be exceptions, and people who vote for TCB or TJS out of preference rather than party affiliation.
TJS contested for SDP. When he contested in PE, all his logistic support (including the contractor who did the TP stadium rally stage) came from SDP. So naturally most of the SDP supporters voted for him. Also good fit between his left-wing ideology and the SDP.
But there's a tacit understanding now between TCB and the SDP, now that he's shaken CSJ and Tambyah's hands at the first and last rallies. With TJS not standing anymore, the bulk of that 25% will now go to TCB rather than the PAP-appointed candidate.