Sen. Sam Nunn (D-Ga.), who last year nearly persuaded the Senate to vote to withdraw some U.S. troops from Europe, is moving to broaden the debate this year with new arguments that NATO's war plans are seriously flawed and need to be revised.

Nunn, respected senior Democrat on the Senate Armed Services Committee, says he intends to hammer away at such questions as:

* Why should the United States continue to spend billions to stockpile enough ammunition to fight for 30 days in Europe when the European allies would run out of bullets in less than two weeks?

* Why should the United States try to fly heavy divisions to Europe, one or two tanks at time, while the Europeans are lightening their divisions?

His legislative options include another attempt to reduce U.S. troop strength in Europe, an attempt to force the Pentagon to restructure the forces destined to go to Europe in an emergency and a reapportionment of the defense budget to provide more money for planes and ships to take troops and weaponry to distant trouble spots.

Nunn fired his first shots at Army witnesses during opening hearings on President Reagan's latest five-year defense plan.

"If you're a Soviet military man and you thought the U.S. Army could fight 75 days, with everybody all around them running out in about 12 days, would that add to deterrence?" Nunn asked Gen. John A. Wickham Jr., Army chief of staff. Once the war started, Nunn said, U.S. units could not dash around the battlefield to pass out ammunition to allied units.

Wickham conceded that in that "hypothetical" situation U.S. forces would be better off if they and their allies had more tanks to fight with in the first days of battle rather than mountains of ammunition. The weaponry, he said, might change the course of battle in the first hours or days so that who won or lost would not be determined by who ran out of bullets first.

Nunn said the big question since shortly after World War II has been whether the United States could fly enough troops and weaponry to Europe in time to help blunt a Warsaw Pact invasion. Sen. Barry Goldwater (R-Ariz.), new chairman of the Senate Armed Services Committee, agreed. The shortage of transport aircraft the United States needs to airlift men and weaponry to Europe in the early days of a war "has been 20 percent since World War II, and there is no way of seeing an improvement," Goldwater said.

Wickham said the reason for the gap is that as the Air Force builds more and larger cargo planes, the Army fields more and heavier weapons.

"Lightness is a state of mind," Wickham said. "We need to look in the Army at lightening up our equipment. One-third of the weight of our ammunition is just wood, the way we have been doing it since the Civil War." The Army, he said, as part of its weight-reduction effort is starting to pack ammunition in light plastic.

The larger effort is light divisions. The Army has decided to organize two light divisions of 10,000 people each out of existing forces, the 10th Division to be based at Fort Drum, N.Y., and the 6th Division in Alaska. Wickham said one C141 transport aircraft would have to make 1,200 trips to move the 82nd Airborne Division to Europe while only 500 such flights would be required for one of the new light divisions.

But this is only one issue among many for Nunn and other congressional critics of North Atlantic Treaty Organization defense posture. "We are not getting anywhere unless we start behaving as an alliance," Nunn told Wickham and Army Secretary John O. Marsh Jr.

This year, as last, Nunn and his allies will argue that unless European NATO partners spend more to build up conventional forces, nuclear weapons would be the only way to stop a Warsaw Pact invasion.

In an effort to force European members of NATO to improve conventional forces and fulfill such commitments as building shelters at airfields for U.S. warplanes, Nunn last year proposed to reduce on a sliding scale the number of U.S. troops in Europe if allied nations did not reach established goals. The amendment was tabled, 55 to 41.

The United States has four Army divisions, two brigades, two armored cavalry regiments and 28 Air Force tactical squadrons in Europe. The Reagan administration's goal is to send enough reinforcements to Europe in an emergency within 10 days to form 10 Army divisions, 88 Air Force squadrons and one Marine amphibious brigade.

Gen. Bernard Rogers, commander of NATO forces, is expected to aid the effort when he testifies before the committee March 1. In past years, Rogers has said that "because of the failure to meet commitments in the conventional area . . . , we have mortgaged our defense to the nuclear response.