As Congress has rewritten it, President Carter's energy plan would not cut U.S. oil imports anywhere near the 4.5 million barrels a day the administration originally proposed, experts in and out of government now agree.

Some industry and Wall Street analysts even say that what is now the main feature of the plan - the section taking price controls off natural gas - could work perversely to increase oil imports.

But administration policymakers deny this. And while they agree that the plan as a whole has been weakened in Congress, they say it still goes a long way toward accomplishing its original objectives.

The main objective of Carter's energy plan from the start was to curtail oil imports, for reasons both economic and diplomatic. Like presidents Ford and Nixon before him, Carter set out to reduce U.S. dependence on oil-exporting countries, particularly in the Middle East, and to stop the flow of U.S. wealth abroad to pay for oil.

To curb imports he decided to try to curb consumption generally by raising prices, mainly through a proposed tax at the wellhead on domestic crude oil. On top of that, he proposed a second tax on industrial users of oil and natural gas, to help persuade them to shift to coal.

These were the two main elements in the original plan. It was said that their combined effect would be to reduce consumption and therefore imports of oil by more than 2.3 million barrels a day, more than half the projected total.

It was only as a kind of necessary corollary that Carter also proposed letting natural gas prices rise (but still keeping them under federal control). The reasoning was that if oil prices went up and gas prices did not, all sorts of users would shift to gas and exacerbate the gas "shortage" in the nonproducing states of the northeast and midwest.

Congress, however, has reversed this. It has refused to pass the oil taxes, but at the same time has voted to let gas prices eventually rise even further than Carter wanted.

Some economists think the result could be to make oil more attractive than gas and other fuels, thus increasing oil demand, consumption and - ultimately - imports.

One such analyst is Arlan Tussig, formerly chief economist for Sen. Henry M. Jackson's Energy Committee. Jackson helped shepherd the gas bill through Congress (it still awaits a final vote in the House).

But says Tussig: "My hunch is that the net effect of the gas bill will be to increase oil imports."

A similar view comes from the Wall Street investment banking firm, Smith Barney Harris Upham and Co., which recently predicted the gas bill could increase oil imports 2 million barrels a day by 1985.

Ted Eck, senior economist for Standard Oil of Indiana, which opposes the gas bill, also says, "The gas bill could result in more oil imports."

The administration, for its part, says it is still too early to be sure what effect the legislation on which Congress is still working might have on future consumption and imports.

The Department of Energy, however, has shifted the focus of its arguments in favor of the gas bill. Originally it said that the main reason for raising gas prices was to save gas. Now it says the main reason is to stimulate added production of gas, which it claims will then displace oil in the energy marketplace, reducing imports.

Whilethe effect of the gas bill is in dispute, almost all experts agree that, without the proposed oil taxes, the plan will be less effective than first proposed in reducing imports. Few think that that in its present form it can save more than half the 4.5 million barrels a day Carter set as his goal.

The remaining parts of the original plan - a standby gasoline tax, a tax on gas-guzzling cars, tax credits for home insulation, coal conversion, utility rate reform and others - have almost all been either knocked out entirely or weakened.

The Energy Department itself has given preliminary estimates to the Senate Energy Committee suggesting the eventual reduction in imports will be in the range of 1.8 million to 2.4 million barrels daily, depending on the final shape of the legislation.

Politically, however, the bill remains as important to Carter and others who have worked for it as ever.

There are two aspects of the plan still in question: One is that the House must still approve the gas legislation; the other is that tax conferees have not yet settled finally on all of their part of the bill. They have, however, pretty conclusively rejected the crude oil tax that the House passed but the Senate did not, and on which most of the tax provisions depended for their full effect.