WASHINGTON  President Obama’s top antitrust official this week plans to restore an aggressive enforcement policy against corporations that use their market dominance to elbow out competitors or to keep them from gaining market share.

The new enforcement policy would reverse the Bush administration’s approach, which strongly favored defendants against antitrust claims. It would restore a policy that led to the landmark antitrust lawsuits against Microsoft and Intel in the 1990s.

The head of the Justice Department’s antitrust division, Christine A. Varney, is to announce the policy reversal in a speech she will give on Monday before the Center for American Progress, a liberal policy research organization. She will deliver the same speech on Tuesday to the United States Chamber of Commerce.

The speeches were described by people who have consulted with her about the policy shift. The administration is hoping to encourage smaller companies in an array of industries to bring their complaints to the Justice Department about potentially improper business practices by their larger rivals. Some of the biggest antitrust cases were initiated by complaints taken to the Justice Department.