New President Sam Church Jr. opened the 48th constitutional convention of the often-fractured United Mine Workers of America here today with a call for unity. $"We must again be the mighty power that we once were," the 43-year-old Church, who succeeded the ailing Arnold Miller Nov. 16, told the convention delegates.

"The issue is unity, a commonality of purpose . . . You must sell the UMWA every day, not condemn it. If one member detracts from this union, that makes organizing that much more difficult," Church said in his keynote address, which he termed "a call to arms."

"The voice of the United Mine Workers must come out as one loud, thunderous voice, not factions on the sideline," Church told the 1,200 delegates attending the 10-day convention. "Lawmakers can easily ignore a few of us, but the voice of thousands of miners must be recognized."

The attitude of many delegats and union officials gathering here is that Church, a former mine electrician from southwestern Virginia, should have a chance early in his term to operate the union free of dissension.

The union witnessed severe internal strife in the reign of Miller, who ousted W. A. (Tony) Boyle in 1972.

Two major convention issues will give delegates a chance to unite behind Church.

One is a proposal for a monthly dues increase from $12 to $25.50 for working miners.

The other is a proposed waiver of the union's constitution that would allow Church to appoint his successor as vice president, rather than call a special election.

The dues increase is needed to meet expenses and start a massive organizing campaign in the mostly nonunion western-mines, union officals say.

"If our organizing in the West is going to be thorough, we must recognize that the cost will be huge," Church said today. He gave no figures.

The proposed constitutional waiver to allow Church to appoint his successor is, in effect, a referendum on his leadership. Union leaders argue that giving him the authority would strengthen his hand in guiding the UMWA and would eliminate the dissension and finiancial costs of a special election.

About 180,000 active and 90,000 retired miners in the United States and Canada could vote in an election.

Delegates say Church is expected to get his way. "I don't see us as having any choice if we're going to stay together," said a Pittsburgh district representative.

Church survived an early test of his leadership today by peremptorily overturning a vote against his credentials committee.

The convention voted 566 to 544 to reject a committee report assigning fractional voting power to delegates. Another vote was ordered.

But Church came to the podium. "Are we going to go on with this, or are we going to seat these deleates?" he asked.

A chorus of "yeas" came from the floor, to which Church responded; "The motion [to accept the committee report] carries."

Asked later if his action was constitutional, Church said: "It must be, the delegation voted for it."

There also was optimism that Church would win approval of a selective-strike fund he contends is needed because of divisions within the Bituminous Coal Operators Association, which bargains for the industry.

The divisions mean that some coal companies might settle before others when they negotiate new contracts with the miners in 1981. The proposed fund is designed to help support strikes against holdout companies.

There were complaints here today that the Carter administration has not done enough to boost coal production or help the 20,000 miners out of work.

"I have to question the logic of an energy program in the U.S. that excludes its most abundant resource -- coal," Church said. "That oil is being used to make electricity is an outrage. . . . There should not be any oil-burning [power generating] plants in the country."

On the subject of unemployed miners, he said: "My anger will not subside until these miners are back to work and there are no prospects for any other massive layoffs."