MCI Telecommunications Corp., which made its mark as an interstate long distance telephone company, is mounting a major campaign to provide long distance service within state lines.

The Washington-based corporation so far has won the right to provide intrastate service in five major markets, four of which it gained access to this year.

MCI yesterday announced that public service commissions in Maryland and Florida granted the company intrastate long distance service rights. The company previously had received permission to carry long distance calls within state lines in New York, Ohio and Texas.

Another MCI application for intrastate service is pending in California, where successful entry would put MCI into a strong position to challenge American Telephone & Telegraph Corp.'s longtime dominance of intrastate calling.

"We estimate that fully half of the long distance revenues generated in California come from intrastate calls," said Howard Crane, an MCI spokesman. "Obviously, we're looking at states where there is the most long distance traffic."

MCI already is AT&T's chief competitior in providing long distance telephone service between states. The Washington company's latest move against AT&T was made possible by the federal court order last year directing the breakup of the giant Bell System.

Under terms of that order, effective Jan. 1, 1984, state regulatory commissions can determine whether to allow competition in intrastate long-distance service. MCI's decision to take advantage of that ruling "is additional evidence of increasing competition" in the domestic telecommunications industry, AT&T spokesman Pic Wagner said yesterday.

MCI has provided intrastate telephone service in Texas since 1974, largely because that state, until recently, did not regulate telephone companies.