House-Senate conferees last night broke a week-long impasse over a bill giving the president standby authority to ration gasoline.

The breakthrough brings the conferees very close to agreement.

In addition to the rationing authority, the bill, a major item in President Carter's energy package, would set in motion a program through which states would develop lesser conservation measures.

The impasse was over the right of Congress to veto a rationing plan proposed by the president.

The Senate wanted the right to veto any gas rationing plan before it was put on the shelf. The House wanted a congressional veto only when it was about to be put into effect, arguing that the House would never approve a rationing plan absent a real emergency.

Under the compromise approved by the conferees, the plan would have to be disapproved within 30 days by both houses for the veto to take effect. But the president could then veto Congress's disapproval, and it would take a two-thirds vote of both houses to override that veto.

If they did override the presidential veto, the president would submit a new plan, and the process would start all over again.

Once approved, the plan would sit on the shelf until an emergency arose the president decided to implement it.

If the emergency amounted to less than a 20 percent shortage of motor fuel and diesel fuel, it would require a concurrent resolution by both houses to put the plan into effect.

If the emergency were more than a 20 percent shortage of gasoline and diesel fuel, the plan would go into effect subject to a one-house veto.

One key issue remaining to be resolved is whether there should be a threshold shortage before the lesser conservation measures would go into effect.

Under the bill the House passed, these would require a 10 percent shortage of petroleum products. Senate conferees argued this trigger was too high, and the president and states should have the right to impose conservation measure with a lesser shortage.

The conference broke up last night with agreement on everything but this issue, Conferees ordered the staff to work out a compromise and Senate conferees said they expected no further formal meetings.

The conservation measures triggered at a certain shortage threshold could include anything but weekend closing of gas stations.

The president would set target amounts to be conserved for the states. The states would then draw up their own plans to meet those conservation goals subject to federal approval. If a state did not meet the goal, a federal plan could be imposed.

The president attempted to get gas rationing authority in May, but was thwarted by the House. In July, the house passed the conservation bill with gas rationing authority in it. The Senate's version of the conservation bill did not contain gas rationing authority, but the Seante conferees accepted the rationing provision and the Senate is expected to go along.