In appointing Patrick J. Buchanan as his White House director of communications, President Reagan is taking on a conservative with a bare-knuckle rhetorical style and the firm conviction that large portions of the media, particularly the television networks and leading daily newspapers, have a strong liberal bias.

Buchanan, 46, left the White House in 1974 after serving as speech writer and adviser to Presidents Richard M. Nixon and Gerald R. Ford, but he has remained visible and vehement in Washington as a syndicated columnist and regular panelist on the television show "The McLaughlin Group" and on the now-defunct radio show, "Braden and Buchanan."

"He's very agreeable on the show," said Jack W. Germond, a fellow panelist on the McLaughlin show. "I call him a full-mooner on the air and it's never seemed to bother him. But he'll probably get me for it now."

Buchanan's appointment introduces a more combative temperament to an administration led by a president whose style is generally to massage the media rather than confront it. And Buchanan's new influence should comfort conservatives who feel Reagan has been subverted by nonideological pragmatists in the White House.

In the past he has advocated invoking the antitrust laws against the networks, subscribed to Reagan's description of the Soviet Union as an "evil empire" and called the welfare state of the last half-century in many ways a "disaster."

As a columnist he holds forth in take-no-prisoners prose.

"What The New York Times and assorted allies are up to now is the discrediting of an administration they could not defeat, the thwarting of a national mandate by the policies of obstruction," he wrote last Friday.

"An ideological bulwark of the Democratic Party, a polemical and publicity arm of American liberalism, the big media are the strategic reserve of the Walter F. Mondale presidential campaign," he wrote during the presidential campaign last year.

Two months ago he turned his sights on Republican pragmatists: "Is there more than a little truth in the jibe that Republicans, once empowered, inevitably become less conservative and more corporatist, that the GOP is always capable of rising above principle to stand upon the sacred ground of maximized profit?"

As White House speech writer, beginning with Nixon's first inauguration, Buchanan was a principal author of then-Vice President Spiro T. Agnew's attacks on the media late in 1969.

Buchanan never wavered in his support of Nixon through Watergate and, at the end of Nixon's tenure, advocated an all-out counterattack by the president. He appealed to the president to "break it off -- who will govern America, them or us?" Of the counterattack, Buchanan said: "If we have to drift into demagoguery so be it -- we owe them a few."

But Buchanan was one of the few top White House aides untainted by the Watergate scandals.

"I don't think Nixon ever told him anything about the cover-up because he knew Pat didn't approve of that stuff," said John Sears, a long-time friend and campaign adviser to Nixon and Reagan. Buchanan refused to lead the investigation of Daniel Ellsberg, who leaked the Pentagon Papers, an investigation that Buchanan argued would be futile.

Buchanan's testimony in the early fall of 1973 before the Senate select Watergate committee on the first day of its investigation of 1972 campaign "dirty tricks" broke the panel's summer-long momentum.

He argued that most of the "dirty tricks" were standard political tactics, with which many veteran senators on the committee agreed. "It was a real blowout and an unmitigated disaster," said one. "He made us look like fools."

Buchanan's association with Nixon began in late 1965, when he cornered the former vice president at a cocktail party in St. Louis where Buchanan was an editorial writer for the Globe-Democrat. He told Nixon he wanted to be in on his presidential race from the beginning.

The direct approach is typical of Buchanan. He grew up in Washington, D.C., in a large, strong-willed, devoutly Catholic family. He attended Gonzaga High School and then graduated cum laude from Georgetown University.

He was suspended from Georgetown for a year after assaulting two policemen. Friends say his marriage 14 years ago to the former Shelley Scarney has calmed his physical pugnacity.

But his renowned combativeness, rhetorical and political, is at the service of the White House once again.