The White House threatened yesterday that the president would veto a $1.7 billion supplemental appropriations bill containing an amendment to take away his authority to "defer" spending, a power that some congressional Democrats describe as a virtual line-item veto.

Presidential spokesman Larry Speakes called the bill an "overstuffed turkey that is loaded with deficit fatteners and bad policy."

The bill, which reaches the House floor today, would disapprove $5.3 billion in deferrals the president has announced, and "even worse, the bill would actually attempt to repeal the president's authority to defer budget authority," Speakes said.

The $5.3 billion in proposed deferrals includes money for the Economic Development Administration, fossil energy research and development, energy conservation, the Strategic Petroleum Reserve, low-income housing and elderly and handicapped housing assistance and Community Development Block grants.

Overall, the president proposes to "defer," or not spend, $13.3 billion appropriated by Congress.

Democrats contend that President Reagan has been using his deferral authority to unilaterally eliminate congressionally authorized spending.

Traditionally, most deferrals have been routine. For example, the administration proposes this year to defer spending $30.7 million on prisons because construction is lagging behind schedule and the money will not be needed until next year. According to Republican officials, 56 percent of the deferrals this year were for "management" reasons, such as the prison case, and the rest for policy reasons.

The policy deferrals are the subject of the controversy. Democrats propose to eliminate the policy deferrals in an amendment attached to the appropriations bill, and Republicans plan to offer a substitute -- placing a lid of $5 billion on the amount that can be deferred for policy reasons.

Although deferrals are supposed to merely postpone spending, Democrats say Reagan has been using deferrals to kill programs he does not like.

The president's deferral authority rests in two acts, the Anti-Deficiency Act of 1921, which is designed to prevent agencies from spending money so fast that they run out at the end of a year, and the Impoundment Control Act of 1974, which was designed to stop former President Richard M. Nixon from "impounding" money the Congress wanted him to spend on social programs.

Under the Impoundment Control Act, if the president wants to eliminate spending, he must propose a recission. If Congress does not approve it within 45 days, the money is released. The president proposed $10 billion in recissions this year, according to Speakes, and the appropriations bill would allow only $29 million of the total.

If the president wants to defer spending, he must send Congress a deferral message. Under the original law, either chamber could pass a resolution ordering the money be spent, and the president had no recourse. In 1983, however, the Supreme Court struck down the one-house legislative veto in an immigration case, INS v. Chadha.

Because of that court case, deferrals today must be overturned by legislation that must pass both houses of Congress and be signed by the president. Because of the difficulty of obtaining the two-thirds majority to override a veto, this gives the president almost a line-item veto, according to Democrats.

The supplemental bill contains "urgent" funds for the Internal Revenue Service, authorizes spending $702 million for embassy security and counterterrorism, $50 million for Northern Ireland and nearly $200 million for fighting forest fires.