The following is my quick analysis of the likely effect of the People's Action Party (PAP) and Worker's Party's (WP) election themes and messages ...
PAP Theme/ Message. The PAP's strategy is to exploit the WP's success in the last GE where the WP won a GRC - this has put substantially more "opposition" in Parliament. As such, the PAP is adopting a theme targeting the self-interest of Hougang residents by "localising" it (i.e. not about having an opposition in Parliament as the WP already has a GRC) and making it about "meeting the needs of residents."
Target Audience Assessment. Using the outcome of the Potong Pasir elections as a reference to understand voter psyche, voters are primarily driven by self-interest. They are however willing to vote opposition if the opposition candidate is (a) credible; and (b) can meet the needs of residents provided it is not too far off the norm (norm defined as the perceived benefits of being a PAP ward). This willingness to accept slightly less than PAP wards is balanced by voters desire for an alternative voice in parliament.
Assessment of WP Theme/ Message. My assessment is that WP is continuing with the age old theme of "being a bastion" for democracy. The danger with this theme/ message is that one of the main conditions which prompted Hougang voters to vote opposition in the previous GE is no longer present. In the last GE, it was commonly believed that the WP had a slim chance of winning a GRC. Hence, Hougang residents continued to vote WP to ensure that at least the WP would be in parliament. Now that WP has a GRC, this need is no longer there and self-interest is likely to prevail.
I believe that the PAP has correctly identified that this election will be won by the candidate who can best meet the needs of residents as the issue of democracy is no longer on the table. For WP to win, my believe is that the WP's primary platform must focus on the self-interest of voters with democracy being a secondary theme. As such, I believe the more effective Theme/ Messages for the WP is to focus on (a) demonstrating how Hougang is on par with or better than the PAP-held wards; and (b) play up the familiarity of the WP with Hougang and hence the WP's ability to meet the needs of the residents. The latter message not only counters the PAP's message, but also plays into basic psychology where people are generally resistant to change.