Republican congressional candidate Elizabeth W. Spencer, trailing in money and endorsements in Maryland's 8th District race, yesterday outlined what she called "serious differences" between herself and incumbent Michael D. Barnes, to counter a widespread perception that her campaign is without a theme.

At a morning press conference, and again at a luncheon debate at the Anchor Inn in Wheaton, Spencer went on the offensive in a campaign that so far has been marked by its civility and kind words. Spencer tried to make Barnes' credibility an issue, accusing him of "a pattern of misstatements" that she said "further delineates our differences."

Spencer unveiled seven new position papers, all headlined "Spencer/Barnes; The Differences," on topics ranging from President Reagan's 1982 tax-increase bill to his Middle East peace initiative. Her criticisms included:

* Barnes'quotation in a local newspaper, in which the incumbent criticized Reagan for initiating his Middle East peace plan "without prior consultation with the Israelis." Spencer said she was told by the White House that the United States "was in continuous contact with Israel and Jordan" before the plan was announced. Barnes later repeated his criticism that "The United States did not consult with Israel on this specific initiative."

* Barnes' vote against the 1982 tax increase bill, which Spencer said contains some important midcourse corrections to ease the federal budget deficit. Barnes said, "You bet I voted against it" because the bill included a medicare tax on federal employes.

* Barnes' vote against the Intelligence Identities Protection Act, which, signed by the president in June, prohibits a publication from naming the names of U.S. covert intelligence operatives abroad. In the debate, Barnes said he did vote for the final bill that emerged from a House and Senate conference committee. What he voted against, Barnes explained, was the earlier House version of the bill that was so broadly drawn it would have prohibited publications from reprinting names already published.

* Barnes' charge, made at a previous debate, that the United States may have given Argentina the impression that it would acquiesce if Argentina invaded the Falkland Islands. Spencer said she was assured by the White House that Reagan personally urged the Argentine president not to invade.

Barnes replied that Reagan did make a personal plea, but added, "He did that the night of the invasion. The Reagan administration came into office cozying up to the right-wing military dictatorships in Latin America" and as a result was "sending signals" that a Falklands invasion would be tolerated.