WASHINGTON—House Republicans unveiled the details of the biggest transformation of the U.S. tax code in more than 30 years, calling for deep cuts in business-tax rates and starting a race to pass the complex legislation by year’s end. (Follow live coverage of the GOP tax plan.)

The plan calls for chopping the corporate tax rate to 20% from 35%, compressing individual income-tax brackets, and eventually repealing the estate tax. The bill’s ambitions—along with the slim Republican margins in the House and Senate—could also be...