The Securities and Exchange Commission prepared an aggressive assault against short-sellers, saying it would temporarily prevent investors from making bets on stock declines in an attempt to stem some of the worst stock-market slides in years, according to people briefed on the proposal.

The SEC was convening a late-night commission meeting to consider several items, some of which could be announced as early as today, a person familiar with the matter said. It's unclear whether the halt will be limited to a certain number...