This Sunday sees the HK LegCo elections. The Economist neatly summarises the contradictions at play:

But even if the Democrats and their allies fall short of a majority, most polls predict that they will win 26 or 27 seats (a gain of some five) and possibly a majority of the total votes cast—which is why the Chinese authorities are so worried this time. Their reaction to such a result would most likely be a continuation of their good cop, bad cop tactics—they are too smart to risk outright repression. While they may try to win over hearts and minds by replacing Mr Tung, they will be doubly determined to reintroduce the shelved anti-subversion bill at an opportune time.

More importantly, success for the pro-democracy camp would reaffirm the view in Beijing that further political concessions would cause Hong Kong to slip away on a path to independence, just as Taiwan has. And that is something the current regime in China can never allow, because it could, Mao forbid, spill over into calls for freedom on the mainland itself.