A top communications union official charged yesterday that the Chesapeake & Potomac Telephone Co. is exerting political pressure to persuade District government officials to buy what he called an "antiquated" phone system for the city's new municipal office building.

George R. Strick, a regional vice president of the Communications Workers of America, said he had received "strong indications" that Delano Lewis, C&P's top Washington executive and a political supporter of Mayor Marion Barry, is "pushing the mayor" to award C&P the telephone contract for the new $40 million municipal building at 14th and U streets NW.

"I don't think it's fair to the citizens of the District of Columbia because it's going to cost the taxpayers money in the long run," Strick said.

Lewis said that neither he nor his company is exerting any political pressure on the city. He said he and company officials are presenting their case to the city.

Lewis said that he had discussed the issue with City Administrator Thomas Downs, but has not talked to Barry about it.

Lewis, who noted that the CWA represents C&P employes, said the system it is trying to sell to the city "is important for the financial viability of the company."

A C&P spokesman said that its system is the "fastest, most reliable and up-to-date communications system available . . . . "

Strick and District government officials said the decision on the municipal building telephone contract has broad implications because it is likely to influence the city's plans to modernize the District government's entire telephone system.

Barry recently said that a contract for a new city government telephone system, which now consists of 20,000 to 30,000 telephones, could cost as much as $20 million. Barry said the city must also consider what impact its decision would have on the rates C&P charges its residential customers here. He said a decision not to purchase C&P's system could force the company to raise residential telephone rates to make up for lost revenue from the city government.

Strick and District government sources said that questions about whether C&P has the best system has spurred intense discussions within the Barry administration.

Responsibility for evaluating proposals for the municipal building and the entire District government telephone system is being handled by the Department of Administrative Services. Last year, the department's director, Jose Gutierrez, solicited telephone bids for the new building and the entire District government.

Gutierrez threw out the bids in January, deciding that the city needed to study further the complex technological and financial issues. But the city has to select a telephone system soon for the municipal building, set to open this summer.

Because the city wants the system for the new building to be compatible with a new District-government system, officials said the contract for the new building has taken on greater importance than its expected $500,000 to $700,000 cost.

Barry announced last week that Gutierrez will become his director of policy and program evaluation and will be succeeded by William Johnson, deputy director of the Public Works Department, who will take over supervising the municipal building telephone contract.

The technical debate over the telephone contract revolves around the merits of the C&P Centrex system as opposed to a telecommunications switching system, called PBX, offered by three other companies seeking the municipal building contract -- AT&T Information Systems, ROLM Corp. and Bell Atlanticom Systems Inc.