Somewhere, no doubt, Democrats are still singing "Happy Days Are Here Again," but for party loyalists here in Michigan, a more fitting anthem might be a sad refrain of "What a Difference a Day Makes."

Until one day last February, Michigan Democrats were sitting pretty for the 1978 elections.

The state's popular Republican senator had announced his decision to retire from politics. The popular Republican governor had dropped some hints that he might do the same. The Democrats had formidable rosters of candidates for both jobs.

Then, on Feb. 10, Gov. William Milliken announced that he would run this year for a third full term. On the same day, Sen. Robert Griffin informed GOP leaders that he had changed his mind about retirement and would run for a third term in the Senate.

The emergence of what the Republicans happily call their "dream ticket" has cast a desultory air over campaigning for the Aug. 8 primary election.

There are plenty of candidates - four men are seeking the Democratic gubernatorial nomination, six are competing for the Democratic Senate nod, and one Republican is challenging Griffin - and they are all spending plenty of money. But the voters seem plenty bored.

The rampant apathy towards the primary may also stem from the voters' concentration on single-issue politics. There will be more than six referenda on the November ballot here, including at least two government-limiting proposals of the Proposition 13 ilk.

To the extent people are talking at all about elections, they are discussing the ballot issues, not the candidates.

The candidates, too, are talking mostly about the referenda, and particularly the tax-cutting plans. Like politicians elsewhere, most of them have jumped, with varying degrees of enthusiasm, onto the Proposition 13 bandwagon.

There are two candidates, though, who are bucking the trend. Zolton Ferency, a veteran liberal seeking the Democratic gubernatorial nomination, and Richard Vander Veen, a former congressman running in the party's Senate primary, have both labeled Proposition 13 a "fraud", and are campaigning hard against the whole concept. Both men are considered underdogs in the respective contests.

No matter which Democrats emerge on top in next Tuesday's primary, they will have to be considered underdogs in the fall campaigns against Milliken and Griffin.

Milliken, 55, who succeeded George Romney as governor in 1969, seems a particularly safe bet for reelection. Hardly anyone dislikes him, and those who disagree with him on specific issues frequently back him anyway because of his reputation for efficiency and honesty.

The governor has been criticized for his indecisive response when Michigan cattle were contaminated by the chemical PBB, and for failing to stop construction of a controversial Navy signal facility in the state's northern peninsula. But neither looms as a giant killer for the Democrats.

The favorite in the Democratic gubernatorial primary is state Sen. William B. Fitzgerald, 36, who has strong support in the Democratic bastion of Detroit but is not well-known in other parts of the state.

The 54-year-old Griffin, who served 10 years in the House before his two Senate terms is more valuable than Milliken to a Democratic challenge. The Democrats can argue that Griffin's 22 years in Congress are long enough. And they can cite Griffin himself to support the argument.

"Twenty-two years is long enough," Griffin said in April, 1977 when he announced that he would not seek reelection this year.

This senator had been disappointed when his friend Gerald Ford lost the Presidency in 1976, and disappointed again when he himself was not chosen Senate minority leader in January 1977. It was time to go home, he said.

For the next 10 months. Griffin took it easy, looking forward to his retirement. He wanted to recruit a Republican successor for his seat, and eventually convinced Rep. Philip Ruppe, a six-term congressman, to make the race.

But all the while, Republican leaders were pushing Griffin to reconsider. The party had backed him for years, they said. Now the senator owed the party one last campaign to keep the Senate seat in the GOP column.

Eventually, Griffin was won over. His change of heart and the resulting announcement of his candidacy for reelection cheered Republicans throughout the state - with a few exceptions.

One exception was Ruppe, who canceled his own Senate campaign when Griffin announced. On the same day, Ruppe canceled his long friendship with Griffin. The congressman has been decidedly cool toward the senator ever since, and has remained conspicuously neutral in Griffin's primary race this summer.

Griffin's Republican challenger, L. Brooks Patterson, 39, is a conservative county prosecutor who has run a spirited campaign linking Griffin to "political bosses" and attacking his absentee record last year. Patterson seems to be more an annoyance than a genuine threat to the incumbent.

Of the six Democrats running for the chance to challenge Griffin this fall, the leader appears to be Carl Levin, a scruffy, casual 43-year-old lawyer who became a fixture in the Detroit media through his repeated battles against the federal bureaucracy while serving on the city council.

Levins support in his home city, however, has been undercut by Vander Veen, a thoughful, low-key Grand Rapids native who captured Gerald Ford's old congressional seat for the Democrats in 1974 by running a strong anti-Nixon campaign.

Vander Veen, 55, lost his seat to Republican Harold Sawyer in 1976, and quickly began preparing for this year's Senate race. He has won support from blacks and labor leaders who praise his loyalty to liberal principles when other politicians are moving to the right.

The new face in the Democratic Senate race is Phil Power, 40, a millionaire's son from Ann Arbor who owns a string of local newspapers. Power will spend about $1 million on a media campaign designed to convince primary voters that only he, as a newcomer to politics, can beat Griffin this fall.

Griffin, for his part, doesn't act worried about any of the Democrats, neophyte or veteran. He laughs off their references to his "retirement" last year, and smiles indulgently at their suggestions that he is too tired to serve another six-year term.

The senator has been campaigning with a vengeance already, crowning beauty queens, downing Vernor's ginger ale and marching in parades all over the states.

"Tired? No, I'm not tired." Griffin said the other day in the midst of a six-city campaign jaunt. "I think this is going to be a very enjoyable year - right through election night."