The U.S.-backed Nicaraguan "counterrevolutionaries" have fallen behind in their timetable for rapid victory over the leftist Sandinista government in Managua and increasingly talk of the need for radical changes in the war if they are to win.

Rebel leaders interviewed here speak of hopes for the start of a genuine popular insurrection against the Sandinistas, similar to the one that overthrew dictator Anastasio Somoza four years ago. But U.S. diplomats asked about that possibility said they consider it highly unlikely.

Failing that, some leaders of the contras, as the insurgents are called, say they hope for direct U.S. intervention.

"It's the less cruel way, with less suffering," said Edgar Chamorro, one of the eight directors of the Nicaraguan Democratic Force.

Even without a direct invasion by U.S. troops, the anti-Sandinistas have shown increasing dependence on Washington, not only for covert funding but for diplomatic and political support vital to their cause.

With obvious hostility to their movement in Congress--as the House vote of 228-to-195 Thursday for a cutoff of covert assistance made clear--and facing the more powerful and disciplined military-political responses from Nicaragua, the insurgents say that time may be running out on them.

"This is not a heroic, prolonged, national liberation movement," said Chamorro, contrasting the war waged by his forces with the classic, long-term development of leftist insurgencies in the region. "We work with timetables." Last spring, as the Democratic Force launched its first large-scale offensive in northern Nicaragua, there was talk of victory before the end of the year and of major strides by mid-summer.

But there have been no such gains. The forces have retreated to camps along the Honduran border, one of the largest of which is inside this country near the village of Las Trojes.

A second offensive, aimed at taking the town of Jalapa in the same region in June, is now described by contra leaders here as an aberration, the action of a single powerful commander known as "Suicide," acting on his own initiative.

"It was like a border war. Not good for us. Not good for Honduras. Not good for anyone," said Chamorro.

Meanwhile, efforts to establish active "task forces" of several hundred men deep inside Nicaragua have also failed, although leaders here say that many of their soldiers remain in place in regions such as Matagalpa.

Where it was once believed that the entry of former Sandinista commander Eden Pastora into the fray--operating independently in the south near Costa Rica--would be a significant factor, his forces have yet to prove themselves capable of operating outside a remote jungle. The support that Pastora expected from within the ranks of the Sandinista military does not appear to have materialized.

The Nicaraguan government says it has lost more than 600 persons fighting the contras. The anti-Sandinistas place their own casualties at about 400, including both dead and wounded.

But the Sandinista forces arrayed against the insurgents outnumber them by at least 5 to 1 and have proved to be effective.

At the same time, the socialist indoctrination, militarization and regimentation that draws heated opposition from Washington and other countries in the region gives the Sandinistas a pervasive political presence and intelligence network throughout Nicaragua.

"The Sandinista infrastructure is not going to be penetrated," said a diplomat in Managua who is personally hostile to the rule there. "They're too capable. They've got everything and everyone infiltrated."

The diplomat repeated a common evaluation, that if the Sandinistas are able to finish out this year they will have so consolidated their strength that nothing short of full-scale war could pry them out of power.

"We need 500 noncommissioned officers we don't have. We need very good logistics and we don't have them. We need urban organizations we don't have," Chamorro said.

Many of these problems are blamed by the contras and their most enthusiastic backers in Washington on insufficient funding. The Reagan administration has put more than $90 million into their activities this year and CIA Director William Casey reportedly has asked Congress for $30 million more to fund them in fiscal 1984, which begins Oct. 1.

"I think some people up around Reagan actually believe with enough pressures the contras can get the Sandinistas out," said the diplomat in Managua.

Emilio Echaverry, military chief of staff of the Nicaraguan Democratic Force, conceded that his troops--who are estimated to number from 4,500 to his own high figure of about 10,000--"find themselves at the moment in a certain static situation."

But the ex-major in the Nicaraguan National Guard argued that pressure from the United States and other countries is "closing the ring" on the Sandinistas, "tightening the political, diplomatic and economic circle."

"We conceived that by the end of the year we would either be on top of our objectives the ouster of the Sandinista leadership or at a distance where it is in sight," said Echaverry. "We believe that as things are developing, that time period will be complied with."

But Echaverry added, "We have to enter into a new phase"--something beyond what he called the "hybrid" mix of guerrilla and conventional tactics currently used for hit-and-run attacks.

"You will note well that we are not employing terrorism," said Echaverry. "That does not mean we don't have the capacity."

He also referred to several light planes the contras now operate out of what they call "clandestine" airstrips, presumably in Honduras.

Echaverry acknowledged that necessary popular support for an insurrection does not yet exist. The Nicaraguan people "still haven't come out of that shadow, those expectations" that came with the revolution of four years ago, Echaverry said, even though very few of those promises have been kept.

The "pressure from the people on the Sandinista system still has not been produced," said Echaverry, but "it will be in the short term."

Anti-Sandinista leaders and some U.S. military analysts say a vital element is what a senior American officer recently called "the mantle of inevitable success," the sense that the contras have such powerful backing and such strong forces that nothing can stop them.

That mantle is currently tattered by the actions of the U.S. Congress and the contras' own failures. But the Reagan administration's commitment of more than 4,000 troops to large-scale exercises here and the arrival of the U.S. flotillas off the Central American coast are seen by the rebels as having done much to stitch it up.