The proposals to restrict funds and force withdrawal produced intense pressure on Nixon to bring an end to the war on his own terms before his legislative opponents gained too much ground. During Nixon's first term, there were 80 roll call votes on the war in Congress; there had only been 14 between 1966 and 1968. In 1971, Mansfield attached an amendment to three pieces of legislation that required withdrawal of U.S. forces nine months after Congress passed the legislation. The White House warned that the president would not abide by this declaration. Congress agreed to pass the amendment but only after deleting the withdrawal date and declaring it to be a sense-of-Congress resolution, rather than a policy declaration, which was stronger. While the Senate had watered down the amendment, the expanding number of votes in support of it made the administration well aware of an increasingly active and oppositional Congress.
In 1972, Church and Senator Clifford Case of New Jersey were able to push through the Senate an amendment to foreign-aid legislation that would end funding for all U.S. military operations in Southeast Asia except for withdrawal (subject to the release of all prisoners of war). Senate passage of the legislation, with the amendment, marked the first time that either chamber had passed a provision establishing a cutoff of funds for continuing the war. Though House and Senate conferees failed to reach an agreement on the measure, the support for the amendment was seen by the administration as another sign that antiwar forces were gaining strength. The McGovern-Hatfield amendment was enormously popular with the public. A January 1971 Gallup poll showed that public support for the amendment stood at 73 percent.
During the final negotiations with the Vietnamese over ending the war, culminating with the 1972 Christmas Bombings and the Paris Peace Accords in January 1973, the president knew that he only had a limited amount of time before Congress finally used the power of the purse to bring the war to an end -- regardless of what the administration wanted. Indeed, to make certain that the president could not reverse course, in June 1973 Congress passed legislation that included an amendment sponsored by Church and Case to prohibit the use of more funds in Southeast Asia after August 15. Sixty-four senators voted in favor. When the House assented, its vote marked the first time that chamber had agreed to cut off funds, too.
Most importantly, Congress passed the War Powers Act in 1973 over Nixon's veto. The legislation imposed a series of restrictions on the executive branch to ensure that the president would have to consult with the House and Senate before authorizing the troops for long periods of time.
http://www.prospect.org/cs/articles?articleId=12438