Four years ago, after the troubled summer in which Richard M. Nixon surrendered the presidency, Democrat Gary Hart was elected senator from Colorado with the help of campaign material that showed his smiling Republican opponent alongside a smiling Nixon. The leaflet was captioned: "They had their turn. Now it's our turn."

After two years of Jimmy Carter in the White House, the shoe is on the other foot in Colorado.

Politicians of both parties chuckled in remembrance last week when they saw the campaign material of Rep. William Armstrong, the anticipated Republican challenger to Democratic Sen. Floyd Haskell.

The 9-by-14-inch leaflet featured Haskell alongside Carter on the president's visit to Denver last may 3. Both men were smiling broadly. Again the caption read, "They had their turn. Now it's our turn."

As one Democrat puts it Carter started out low and has been sinking ever since in Colorado where his water, timber and rangeland policies are viewed as distinctly antiwestern.

Haskell's problem of being reelected in this context has beencompounded by the emergence of fiscal issues favorable to Republicans, such as the state spending limitation that will be on the November ballot. Potential opponent Armstrong, a conservative who has run ahead of his party in three successful campaigns for the House, has hammered away at Haskell throughout the summer on issues of inflation and budget balancing. He portrays the incumbent as a "big spender."

Also, Carter did Haskell little good on his trip to Denver last May when he called the senator "a national treasure." Haskell's critics, who consider the metaphor overblown at best, have kept it derisively alive.

But while Carter is a cross to bear for Haskell, the president inadvertently may have become the political salvation of Democratic Gov. Richard Lamm.

Two years ago, after a series of blunders, Lamm was seen as an off-course governor who would be a push-over for the Republicans this year. However, the celebrated White House "hit list" of federal water projects, which included at first five and ultimately three Colorado projects, gave Lamm the opportunity to become the politican who would stand up and speak out for Colorado.

He had made the most of it. Bluntly opposing Carter policies, Lamm has compared the national administration to "a plague of locusts" on the West. Sometimes he begins his political speeches by praising an unnamed Carter who he says really isn't so bad after all.

"It's his brother in the White House that I'm worried about," is the punch line.

Lamm's conduct is frustrating to the two Republican candidates for governor, state Sens. Ted Strickland and Richard Plock, who complained that the governor takes political advantage by attacking Carter administration in the war for Colorado water," Strickland declared last week.

However, even Republican politicans who publicly denounce Lamm for vetoing a state air pollution control bill say privately that he has won public administration for taking a stand against Carter.

Lamm now is favored to defeat either Strickland or Plock, who are locked in a bitter battle for the Republican nomination that will be decided in the Sept. 12 primary.

Under Colorado's two-step candidate selection system, Strickland won by far the most delegate votes at the state GOP convention and the top line on the primary ballot. He has a conservative voting record, impressive strength among the state's Baptists (he is an active member of the church) and a top political organizer in Natalie Meyer, who formerly managed Colorado campaigns for Armstrong and Ronald Reagan.

There is an edge of intensity to Strickland that makes some listeners who see it as zealotry uncomfortable, but he is generally considered a superior campaigner to Plock, an amiable and intelligent Denver lawyer who in his posters and television commercials bears a vague resemblance to Nixon.

Plock, the Senate majority leader, has been portrayed by Strickland as being moderate or liberal, although both have conservative voting records. A "fact sheet" put out by Strickland was denounced Thursday by Plock as "shoddy, misleading and distorted."

In the U.S. Senate primary race, Armstrong has a comfortable lead over ex-astronaut Jack Swigert, a pleasant nonpolitician whose campaign has failed to reach orbit.

Swigert guided the crippled Apollo 13 spacecraft to a safe return in 1970, a feat his campaign literature credits to "ingenuity, teamwork, some incredibly efficient technology and the Grace of God."

Swigert's achievement has been called to public view through the talents of Baltimore political consultant Robert Goodman, who has produced a television commercial featuring a jumpsuit-clad Swigert outrunning an airplane to the tune of a catchy song, "We Can Walk on the Moon If We Want To."

Goodman made a reputation with such jingles ("My Kind of Man, Ted Agnew Is"), but his lyrics and visual effects have been surpassed this time by Roger Ailes of New York, who has produced a number of deliberately unslick television ads designed to make the somewhat formal Armstrong appear relaxed and folksy. The best is a 5-minute spot featuring Armstrong's attractive family-an intended contrast to bachelor Swigert and divorced Haskell.

Swigert is so committed to his political undertaking that he has spent his $127,000 life savings on his campaign.

Despite this personal involvement and the fund-raising help of such earthbound Republican stars as Bob Hope and Glen Campbell, Swigert has been outspent, outorganized, and outmaneuvered by Armstrong, who is as much at home in the world of politics as his opponent is in space.

Swigert, former staff director of the House Science and Technology Committee, has the second line on the primary ballot and probably has squandered his political energies by visiting all of Colorado's 63, counties rather than building an organizational base in his home city of Denver.

At first, some Republicans thought Swigert would emerge as a moderate alternative to Armstrong, but he has avoided that role. In an interview last week Swigert said he was more conservative than Armstrong on some issues. As an example, he said he supported United States withdrawal from the strategic arms limitation talks.

Most of Swigert's statements have been focused on such nonpartisan questions as disposing of nuclear waste by shooting it into "black holes" in space. In contrast, Armstrong talks repeatedly about specific legislation he says is needed to cut taxes and reduce federal spending.

Armstrong faces a tougher race against Haskell, a former Republican who once served with him in the state legislature. Like Armstrong, Haskell is a canny politican who knows how to use the values of incumbency.

Haskell also is well aware that challengers have a way of sneaking up on incumbents in Colorado, as he did on Republican Sen. Gordon Allott six years ago. Perhaps because of the late primary, challengers usually surge in the polls after it is over-as Haskell did and as Hart did four years ago against Republician Sen. Peter Dominick.

With this in mind, the Haskell campaign has begun running television and radio commercials aimed at the general election.

"We are running as if we had a primary," says Haskell campaign manager Jean Galloway. "The senator is well aware that Gordon Allott may have been overconfident. We are not. We anticipate a close race."

Haskell, who led Armstrong by 8 percentage points and Swigert by 16 in a June poll taken for him by Patrick Caddell, is widely regarded as the most vulnerable Democratic incumbent in the Senate. His campaign has attraced outside interest from all quarters, including labor unions and Ralph Nader in Haskell's behalf and conservative political action groups and former treasury secretary William E. Simon for Armstrong.

The Haskell forces are preparing to portray Armstrong as a right-winger who is too conservative for Colorado. In the view of the Armstrong campaign Haskell is an out-of-touch and ineffectual liberal who is tied to big spending policies and President Carter.

Both views are carleatures at best. Armstrong's language and manner are moderate and he has demonstrated his independence on such issues as the strip-mining control bill, where he was one of 12 Republicans who voted to override President Ford's veto. Haskell has differed with Carter on many issues, has fought for western water projects and has made a reputation as a tax expert in Washington and Colorado.

But the hardest road is faced by Haskell, who must convince Colorado's voters, as Dominick failed to four years ago, that he is unencumbered by the policies and prejudices of that man in the White House. Now, says Bill Armstrong, its's his turn.