LEWISTOWN, MONT. -- When Rep. Pat Williams (D-Mont.) returned to his home state earlier this month to campaign for a seventh term in the House, his first stop was a Chamber of Commerce candidates' forum in this central Montana farming and ranching community 100 miles inside the district of the state's other House member, Republican Ron Marlenee.

Williams's foray here illustrates a major subtext in this year's contests for House seats -- the knowledge that two years from now comfortably familiar political terrain could be radically altered after redistricting based on the 1990 census.

As a result, Williams and many of his colleagues are positioning themselves for potentially more competitive contests in 1992, when incumbents will be thrown into unfamiliar territory and in some cases pitted against each other.

That is likely to be the case in Montana, which preliminary census figures show will fall just short of enough population to preserve its two seats in the House. As a result, in 1992 Williams and Marlenee -- a liberal Democrat and a conservative Republican whose sharp differences in style and politics reflect the east-west split that makes Montana almost two different states -- are likely to find themselves going head to head in a statewide campaign covering 145,000 square miles with a population of almost 800,000 people.

Williams is hardly alone as he looks ahead to 1992. Many of his colleagues, and many congressional challengers as well, are embarked on what are essentially four-year campaigns. They are fattening their campaign war chests beyond what is necessary for 1990, trying to build their victory margins this fall so they appear as politically formidable as possible heading into redistricting, and cultivating the state legislators and governors who will draw the new congressional boundaries.

"Obviously, 1992 is the big year we've all talked about," said Edward J. Rollins, co-chairman of the National Republican Congressional Committee. "Everybody in this cycle is really running a three-year effort."

"People are saying, 'I'm going to look strong and be strong,' " said Tom King, a political consultant who advises Democratic House candidates. "People who haven't run professional campaigns before are doing it now so they look stronger and there's no vulnerability when the lines are drawn."

The early political jockeying for 1992 is most acute in those areas that are expected to undergo major changes in congressional district boundaries because of population shifts. In general, the process will add congressional seats in Sun Belt states and take them away from the industrial Northeast and Midwest.

In Texas, for example, Rep. Martin Frost, a Dallas Democrat who has not had a major party opponent since 1986, has raised more than $500,000. Unopposed for a seventh term, he has formed a state political action committee that has raised an additional $50,000 and contributed half of it so far to state legislative candidates who will play a major role in redistricting.

Frost's largess -- as well as what one colleague called his effort to "hand-grenade" a campaign finance reform bill this year -- is understandable. With Texas expected to gain three House seats in 1992, Frost will lose some of the strongest Democratic parts of his district because of the creation of a majority black district in Dallas. Both he and his Democratic colleague from Dallas, Rep. John Bryant, will find themselves with more suburban, and more Republican, constituencies.

"I'm trying to do everything I can to be ready," said Frost.

In upstate New York, freshman Republican Rep. Bill Paxon is also maneuvering furiously to strengthen his position in advance of redistricting. It is widely expected that Paxon will draw the short straw when his region loses one of the three western New York state districts likely to be eliminated by redistricting.

Seeking to improve his odds, Paxon already has contributed $20,000 from his campaign fund to a state GOP political action committee organized to preserve the Republican Party's narrow majority in the state Senate, the only bulwark against complete Democratic control of the redistricting process. Meanwhile, mail from Paxon's congressional office has been turning up with some frequency in the districts of at least three of his neighbors, Democratic Reps. John J. LaFalce, Louise M. Slaughter and Henry J. Nowak.

Paxon said the errant mail is an understandable accident in an area crowded with elongated congressional districts that cut erratically across county lines. But some of his annoyed colleagues don't buy it.

"Come on!" said LaFalce when asked if the mailings might be accidental. "I've had lots of it in my district."

Yet another example is in Michigan, where Rep. David E. Bonior, the Democrats' chief deputy whip, is preparing to spend up to $1 million defending his seat northeast of Detroit. Detroit's two black inner-city districts will be protected in redistricting, but the Detroit region as a whole is likely to lose one seat.

Bonior knows that he may end up competing in that area with several other entrenched House veterans, including two powerful Democratic committee chairmen, John D. Dingell and William D. Ford. Bonior's district, which he carried with just 54 percent of the vote in 1988, already is considered marginal Democratic territory so he is trying to bolster his victory margin this time in anticipation of 1992.

In contrast to lawmakers in Texas, New York and Michigan who can only guess what their districts will look like in 1992, Williams knows exactly what he faces in Montana two years from now: a tough campaign across an entire state that would stretch from Chicago to Baltimore if set down in the East, and an electorate far different from the one he has represented since 1978.

With one of the strongest labor traditions of any district in the country, Montana's mountainous 1st District, anchored by the mining city of Butte and the university city of Missoula, has always reelected Williams -- an unreconstructed New Deal liberal who believes passionately in government spending for social programs -- with healthy margins.

The state's 2nd District -- and its representative, Marlenee -- could hardly be more different. Though about half of the population lives in and around the state's two largest cities, Great Falls and Billings, the rest is spread across the vast ranchland and wheat-growing areas of the eastern two-thirds of the state. A gruff, blunt-talking rancher who distrusts government and calls environmentalists "prairie fairies," Marlenee almost never sees eye to eye with Williams.

"One thing about this race," said Marlenee about 1992, "we both represent vastly different ideologies and neither one of us is inclined to change our spots."

Though Williams wonders whether he would have to moderate his views if he wins in 1992 in order to better represent a more conservative electorate, he is not trimming his sails in this campaign. During the swing through his district, he went out of his way to defend his high-profile role as an ally of the National Endowment for the Arts against the attacks of those who accuse the agency of funding obscene art.

Hounded by a small group of pickets carrying signs reading "Porno Pat," Williams told the Montana Arts Council that "people must be free to create." But Williams, who chairs the subcommittee with jurisdiction over the NEA reauthorization bill, also stressed the economic impact of the arts in Montana.

If Williams has changed at all in anticipation of the 1992 race, it is in how he is campaigning. He plans to double his television budget since the last time he used TV and to triple his direct mail effort.

And he has redesigned the logo for his campaign buttons and literature. The 1988 version had the message "Pat Williams/Democrat" superimposed on a map of the 1st District. This year's version says "Montana's Pat Williams/Democrat" spread across a map of the entire state.