WASHINGTON — The Senate on Tuesday overwhelmingly approved a measure halting purchases to the nation’s emergency oil supply, defying President Bush who opposes the idea.

Republican Sens. Olympia Snowe and Susan Collins approved the measure to suspend purchases to the Strategic Petroleum Reserve until oil prices fall to $75 per barrel. Prices have surpassed $120 per barrel.

Proponents of the measure argue that suspending purchases to the emergency reserve would add 70,000 barrels to the open market, increasing supply and driving down prices.

The U.S. Energy Information Administration has estimated the impact on gas prices in the U.S. at between four and five cents a gallon. The average price of a gallon of gasoline stood at $3.72 on Monday.

The amendment passed by a 97-1 margin.

Reps. Tom Allen and Michael Michaud, both Democrats, will get their chance to vote on a similar measure in the House later today.

President Bush has opposed the measure, arguing that the reserve is for national security and natural disaster crises. The reserve, located in caverns in Louisiana and Texas, can hold up to 727 million barrels of oil and it now has 703 million barrels, a capacity of 97 percent.

Snowe and Collins opposed a separate Republican measure that would have halted purchases to the reserve and would have allowed drilling in the Arctic National Wildlife Refuge and off the coasts of states. Drilling is banned in those areas.

The Republican-backed amendment failed 56-42, falling short of the 60 votes it needed to pass.

Six Republicans, including Snowe and Collins, broke ranks and opposed that measure.