WASHINGTON—Merchant groups are forming a national coalition to campaign for stricter antitrust laws, including measures that could force Amazon.com Inc. to spin off some of its business lines.

The effort is being launched Tuesday by trade groups that represent small hardware stores, office suppliers, booksellers, grocers and others, along with business groups from 12 cities, organizers say. Merchants plan to push their congressional representatives for stricter antitrust laws and tougher enforcement of existing ones.

The groups, which collectively represent thousands of businesses, want federal legislation that would prevent the owner of a dominant online marketplace from selling its own products in competition with other sellers, a policy that could effectively separate Amazon’s retail product business from its online marketplace.

Members of the House Antitrust Subcommittee are considering legislation along those lines as they weigh changes to U.S. antitrust law, though no bill has yet been introduced.

The merchant groups also want tougher enforcement of competition laws and legal changes that would make it easier for the government to win antitrust lawsuits against big companies.