TIME RUNS SHORT FOR PHNOM-PENH

Like a battering ram pounding an already crumbling citadel, the Khmer Rouge insurgents last week aimed new blows against the tottering Khmer Republic. On all of Cambodia's battlefields, government troops were falling back or launching unsuccessful counterattacks. Since the rebel offensive began in January, 5,000 of President Lon Nol's troops have been killed and 15,000 wounded. Rebel casualties are estimated to be higher, and civilian losses are considerable.

The most intense insurgent pressure remains concentrated against besieged Phnom-Penh. With the Mekong River lifeline choked off, the capital is now solely dependent on the U.S. "rice birds"-- DC-8s and C-130s whose pilots brave Khmer Rouge rockets to ferry in food, fuel and ammunition. Money for the airlift will be exhausted by the end of April unless the U.S. Congress, when it reconvenes April 7, surprises everybody and approves a $222 million supplemental Cambodian aid appropriation. Last week the strategically important town of Tuol Leap, only six miles to the northwest of Phnom-Penh's Pochentong Airport, fell into rebel hands for the third time since the start of the offensive. That put the airfield within range of the highly accurate U.S.-made 105-mm. howitzers that the rebels have captured. Constant shelling of the runway in mid-March forced the U.S. to suspend cargo flights for two days. With the insurgents once again zeroed in on the airport, it may be impossible for the U.S. to keep the supply line open.

Government soldiers tried and failed to retake Tuol Leap.

They also were slowed in their drive against other parts of the "rocket belt." Two infantry battalions temporarily refused to participate in the attack last week, complaining that they lacked food and that their officers were using poor tactics. They would have faced a formidable barrier around the rebel rocket sites: antitank mines and rockets, antipersonnel mines and machine-gun nests. So far, the best the government has been able to do is sneak observers forward near the rocket belt; when they hear the whoosh of a missile leaving its tube, the observers push a button that triggers warning sirens at Pochentong Airport and in the capital. Insurgents also broke through a small section of the North Dike Road, the last line of defense before Phnom-Penh's northwestern suburbs and the airport.

Elsewhere in Cambodia, the story is just as grim. Several towns on the Mekong River are still under pressure, while even Battambang city, 160 miles northwest of Phnom-Penh in the heart of what was once Cambodia's rice granary, might soon fall. Each night the Communists overrun another tiny outpost protecting the city. An inspection of Battambang's defenses, says a recent visitor, turned up "empty holes and no soldiers to fill them."

Perhaps only a change of political leadership can spare Phnom-Penh eventual strangulation and all of Cambodia even greater bloodshed than it has suffered so far. At week's end, there was some new hope that such a change may still be possible; U.S. sources in Phnom-Penh reported that Lon Nol and his family would soon depart Cambodia for Indonesia and then proceed to the U.S. -- probably Hawaii, where he underwent medical treatment in 1971 for a stroke. This could open the way for a new government and a negotiated peaceful transition of power to the insurgents. Both the Communists and Prince Norodom Sihanouk, the exiled former Cambodian chief of state who is titular head of the insurgents, have vowed never to sit down with Lon Nol.

For the past two weeks, pressure has been continually building on Lon Nol to resign or at least leave the country. The majority of his generals have been increasingly open in urging the President to go. So have been most of the cabinet and nearly all members of the bicameral National Assembly. An ad hoc coalition of top Cabinet and military leaders, in frequent audiences with Lon Nol, have been trying to convince him that his continued presence in the capital could lead to an eventual bloodbath.

If Lon Nol remains adamant and refuses to leave, it is in part because he has ensconced himself in his tightly guarded

Camcar Mon Palace and is surrounded by toadying sycophants who have encouraged him to hang onto power and shield him from bad news. A dialogue with the President has often assumed a sense of unreality. TIME Correspondent David Aikman reported this recent exchange between Lon Nol and Lieut. General Saukam Khoy, president of Cambodia's Senate and Lon Nol's most likely successor:

Saukam Khoy: Well, Mr. President, everyone says you should step down.

Lon Nol: We should stay and fight!

Saukam Khoy: Where do we get the ammunition to do this?

Lon Nol: You should know. You're a military man.

Despite the talk that he may leave the country, there was some evidence indicating Lon Nol may make a last-ditch political stand. This apparently is behind last week's decision by Lon Non, the President's ambitious and ruthless younger brother, to resign from a top army command and seek the post of secretary-general of the Social-Republican Party, the rightist backbone of Lon Nol's political support. Lon Non won a reputation for brutality when, as head of the national police, he violently suppressed student demonstrations in 1973.

If Lon Nol decides to hang on, it is because he hopes his forces can defend Phnom-Penh for three more months, until the wet season impedes the insurgents' drive. With an improved military situation, the President would expect political pressures to diminish. On the other hand, if he finally makes up his mind to leave, it may in no small measure be due to Cambodia's students. Last week, as rumors swept the capital that time had run out for the President and that a coup was imminent, leaders of the Association of Students of the Khmer Republic, which claims 20,000 members, held a press conference. Known as uncompromising opponents of Lon Nol, the students demanded dissolution of the National Assembly and the convening of a "National Congress" in April, at which -- according to them -- Cambodians would be able to "express their true views." Many of the student leaders denounce the U.S. for its continued support of the regime and for supplying the arms that prolong the war (and endanger their draft deferments). In any event, the students' message had to be profoundly unsettling to Lon Nol. The 1970 coup against Prince Sihanouk that brought the present regime to power was, after all, triggered by student demonstrations.

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