The leader of the Democratic Alliance, Helen Zille, has a baited trap ready for the Independent Democrats of Patricia de Lille: she will set it on Saturday when she addresses the ID's special conference in Cape Town.

Zille is to offer to merge the two parties to fight next year's local elections. The bait will be the electoral strength of Zille's DA put at the service of the ID's candidates.

But there will also be a benefit for the DA: the mainly white party will grow a shade or so darker, which has some electoral advantage in South Africa today.

This was not mentioned in Zille's weekly online letter on Friday where she extols the virtue of merger over coalition.

When Zille put together a seven-party coalition to govern Cape Town, the ID was not part of it, as a majority of its councillors had preferred to side with the ANC.

"This proved to be a politically costly decision and the ID was punished by its voters," Zille said on Friday.

"During the months that followed, I began to have regular meetings with Patricia de Lille, the ID leader. We developed a sound mutual understanding and respect. Within a year, it proved possible to reconstitute the Cape Town coalition to include the Independent Democrats."

Zille recalled that early in 2008, she approached De Lille and asked her to consider a merger, behind a common manifesto, and go into the 2009 general election together.

De Lille would be the premier candidate for the Western Cape.

It didn't happen and the DA went on to win alone.

But Zille now says her party is ready to take the next step in preparation for the 2011 local elections.

"That is the background and context that will shape the ID's conference on Saturday," Zille said.

"It is an important event. It should take us one step closer to ensuring the success of South Africa's democratic project.

"The ID faces a unique opportunity. And I'm sure they know it."

The opportunity is, of course, to disappear as an independent political entity, and disappear into the DA's capacious grasp.

The same opportunity is also being offered to other small parties, who will also be at Saturday's conference. But the ID is likely to be the ripest plum to fall into Zille's apron.