Here's some good news for a Monday:

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.

Ms. Varney is expected to say that the administration rejects the impulse to go easy on antitrust enforcement during weak economic times.

She will assert instead that severe recessions can provide dangerous incentives for large and dominating companies to engage in predatory behavior that harms consumers and weakens competition. The announcement is aimed at making sure that no court or party to a lawsuit can cite the Bush administration policy as the government’s official view in any pending cases.

In the speeches, Ms. Varney is expected to explicitly warn judges and litigants in antitrust lawsuits not involving the government to ignore the Bush administration’s policies, which were formally outlined in a report by the Justice Department last year. The report applied legal standards that made it difficult to bring new cases involving monopoly and predatory practices.

As a result of the Bush administration’s interpretation of antitrust laws, the enforcement pipeline for major monopoly cases — which can take years for prosecutors to develop — is thin. During the Bush administration, the Justice Department did not file a single case against a dominant firm for violating the antimonopoly law.