IF PRESIDENT Ronald Reagan decides to propose building the multibillion MX missile system to Congress this summer, be prepared for an advertising and "educational" campaign that may rival that of McDonald's and Calvin Klein jeans.

Smith and Harroff, a Washington public relations and political consulting firm, have already surveyed public opinion in those states affected by the missile project. "There's an awful lot of misinformation on the MX. A number of contractors of the MX have been communicating with us to get the real story out," Mark Harroff, a partner in the firm, said last week. He described the P.R. firm's plans so far as "very preliminary."

Up until now, Smith and Harroff's most famous political clients have been Rep. Tom Evans (R-Del.), Rep. John Rhodes (R-Ariz.), Rep. Marc Marks (R-Me.). Corporate clients who have used Smith and Harroff's public relations expertise include the Committee for Energy Awareness -- a group formed by 200 utility and nuclear reactor companies, Westinghouse Electric, the Edison Electric Institute and the Glass Packaging Institute.

Both Smith and Harroff's tentative plans include advertising, especially in Utah and Nevada where the system would be based, and press briefings for reporters. "If the administration decides to go with it, it will be an ongoing story for a couple of years," Harroff said.

The MX advertising campaign will not be under a government contract, Harroff said. "The government is not paying for a penny of it."

Harroff said if they are hired by one or more of the MX contractors, they will be paid out of corporate funds. It's still very unclear who will be hiring us . . . We've discussed a variety of options but haven't decided on what we want to do," Harroff said.

GTE's Sylvania Systems Group, one of about 30 government contractors working on the MX missile system, has a $325 million contract to develop the MX command, control and communications systems, a spokesman for the company said last week. They "presently have under consideration" hiring the public relations firm if Reagan approves the MX system. Smith and Harroff were hired by GTE's Sylvania Systems Group last November 1980, to do public opinion surveys in Nevada and Utah and finished the contract at the end of March.

A Martin Marietta Corp. spokesman said it had "no plans to participate in such advertising campaign" but added, "some of our aerospace people are aware of some preliminary conversations on an advertising effort."

The latest version of the MX system calls for 23 "shelters for each missile on a straight road track with 200 missiles in all, according to Hecker. Using the elaborate pea-under-the-shell principle, the government estimates the current cost is $33.8 billion in 1980 dollars, but congressional opponents have charged that cost overruns could push the final pricetag to $100 billion.

When Mark Fowler, the chairman designate of the Federal Communications Commission told James C. Miller III the other night he should keep a high profile in Washington, Miller replied "I try to keep a low profile because I talk too much."

Miller is one of the key Reagan Administration officials these days because of his job as executive director of the President's Task Force on Regulatory Relief -- the group that is trying to "get the government off our backs," as President Reagan says. Miller also is the Office of Management and Budget's administrator for information and regulatory affairs under director David Stockman. Miller fondly says of Stockman, "He's the smartest S.O.B. I know." When Stockman was asked recently at a dinner what would happen if all his projections on unemployment and inflation turned out to be wrong when run through computers, Miller leaned over to friends and said, "Dave doesn't need a computer -- he's got one in his head."