Passage Summary: Writers try to change people's minds about who to vote for, but it doesn't work because people have usually already picked their candidate by the time they read the article.

Conclusion: Political columnists are rarely successful in their attempts to persuade voters to change their minds.

Reasoning: By the time a political column is published, the vast majority of voters have already decided which candidate they will support.

Analysis: The argument assumes a 'Gap' between making a decision and being persuaded. It takes for granted that once a voter has made a choice, they are no longer open to changing it based on new information. To find the necessary assumption, ask yourself: 'What if voters frequently changed their minds after an initial decision?' If that were true, the columnist's timing wouldn't matter, and the argument would fall apart.