As an essential element of republican government, Congress possesses the power of the purse. It uses that authority to control presidents and executive agencies. From the start, however, there has always been tension between the two elected branches, with presidents insisting on exercising control over departments and agencies. A major collision occurred during the 1970s, when President Nixon claimed constitutional authority to refuse to spend appropriated funds (the impoundment dispute). He lost that battle in Congress and in the courts. Other issues include covert spending, Gramm-Rudman, Iran-Contra, item vetoes, legislative vetoes, and efforts in recent decades to control the national debt.