Two of the nation's largest accounting firms are combining forces. Clifton Gunderson and LarsonAllen, both among the 20 largest firms, plan to merge effective Jan. 2.

The combined firm, which will be named CliftonLarsonAllen, will employ more than 3,600 professionals, including 500-plus partners, and will operate from 25 states and Washington, D.C. The firms have combined revenues of more than $500 million.

The new firm's specialties will include financial services (including experience with credit unions) and not-for-profits as well as audit, accounting and advisory services.

The new firm will have a holding company and three entities: CLA LLP, CLA Wealth Advisory and CLA Outsourcing.

Clifton Gunderson CEO Kris McMasters will oversee CLA LLP, the public accounting practice. LarsonAllen CEO Gordy Viere will oversee the CLA holding company, the wealth advisory practice and the outsourcing practice, the companies said.