As the Philippines' presidential election campaign begins to warm up, a contest of vastly different styles is emerging between President Ferdinand Marcos and opposition candidate Corazon Aquino.

On one side is a creaky but still powerful political machine run by an aging leader who has ruled the country for the past 20 years. On the other is a self-described "ordinary housewife" who admits she has no political experience but believes she has something more important to offer: "sincerity."

It is an election campaign that one diplomat likens to a contest between Genghis Khan and Joan of Arc.

Regarded even by his foes as a brilliant politician, Marcos, 68, has gone on the offensive in recent days to attack his opponents with lengthy arguments suggesting that they are naive on political and economic issues, would lead the country into instability and risk turning over power to Communist rebels. The reply of Aquino, 52, the widow of assassinated opposition leader Benigno Aquino Jr., is that while she may be inexperienced, the country needs a leader who is the complete opposite of Marcos.

Nowhere has the contrast in styles been more evident than in a series of rallies that began Thursday to open the presidential campaign. The election, called by Marcos more than a year before his term is due to expire, is set for Feb. 7.

After a slow start during a campaign trip south of Manila to the province of Batangas, the home turf of opposition vice presidential candidate Salvador Laurel, the Aquino campaign appeared to pick up momentum today with a large and lively rally in the Philippine capital.

About 12,000 persons gathered in a square in front of the central post office to hear Aquino and Laurel launch their campaign in Manila after a two-hour opposition march from a Roman Catholic church where the devoutly religious Aquino attended mass. The festive crowd waved yellow banners proclaiming Aquino's candidacy and repeatedly chanted her nickname, "Cory," as she and other opposition leaders condemned the Marcos government.

Several politicians, including Laurel, gave speeches in the traditional style of the Philippine hustings: fiery oratory mixed with deprecating jokes against the opponent. But Aquino, the nonpolitician in a plain yellow dress, never raised her voice as she delivered her low-key indictment of Marcos.

"I'm very different from Marcos," she said. "I'm not a politician. I don't know how to tell a lie or take advantage of others. I'm not a dictator. I thank God I'm really very different from him, because if we really want to save the country we need a leader very different from Marcos."

She appealed to Filipinos to "join me in my crusade for truth, justice and freedom. I offer you my sincerity," she said, "and I will do my best to bring about the changes we all have been fighting for."

The crowd periodically interrupted her speech to roar its approval of the widely admired widow's populist message and chant her name. The rally ended with a popular folk singer's rendition of a patriotic song as the crowd raised clenched fists or made an "L" sign with thumbs and forefingers, the symbol of her late husband's political group that stands for the Tagalog word for "fight."

It was a spontaneous outpouring of sentiment by a largely middle-class gathering after what opposition leaders conceded was a disappointing turnout in Batangas. Gone today were the red flags of most street demonstrations in Manila recently by leftist student and labor groups. In their place were yellow banners, T-shirts and a variety of other "Aquino for president" paraphernalia.

Yellow has been the color of the Aquino movement since the slain opposition leader's ill-fated return to the Philippines in August 1983, when a large crowd waited to greet him at Manila International Airport with yellow ribbons symbolizing homecoming.

By contrast, the opening rally of the Marcos campaign Saturday and an earlier party convention that formally proclaimed his candidacy both appeared contrived affairs, the products of political machinery that still works, but without much spontaneity or enthusiasm.

A progovernment newspaper columnist noted as much the day after Marcos' candidacy was proclaimed officially on Dec. 11. Convention fever of past political exercises here was "sadly missing," he wrote, as Marcos' ruling party "went through the motions of holding a convention." About 30 members of a youth organization -- some of whom later said they had been paid to attend -- periodically chanted the campaign slogan, "Marcos pa rin," meaning roughly "It's still Marcos." But few delegates joined in.

At a rally Saturday at the Lipa city hall in Batangas Province, Marcos drew a crowd of about 1,500, including an entourage and supporters bused in from Manila. Despite free drinks, boxed lunches and T-shirts as incentives for attendance, most residents ignored the hoopla.

The main reaction of the crowd came when Marcos and his running mate, Arturo Tolentino, engaged in some locker-room humor at the expense of the opposition. Tolentino, 75, boasted repeatedly about his virility with allusions such as "mature coconuts have more milk." His opponent, Laurel, is 57.

Speaking in Tagalog, Marcos told the crowd it was "kind of embarrassing to be running against a woman." He elaborated with off-color remarks alluding to his opponent's sex.

As his wife, Imelda, sat on the stage behind him and occasionally blushed, Marcos added that he wanted to talk to Aquino. "But it would not be a debate," he said. "It would be a friendly conversation. Of course, I would have my wife as a witness."

Aquino's brother-in-law, Agapito (Butz) Aquino, today dismissed Marcos' remarks. "Cory is above that," he said. "People are not voting for a man or a woman, but for a symbol."

Another Aquino adviser called the statements "desperate moves because Marcos can see a groundswell building up."

While statements like Marcos' undoubtedly would draw howls of protest in the United States, they seem to pass here in a society steeped in Latin-style machismo in which sexism is not a big issue.

Another major difference from modern American politics is that cheating is largely taken for granted in elections here. Opposition politicians say that despite Marcos' denials, his ruling party will try to cheat as much as necessary to keep him in power and that it is the opposition's task to minimize the fraud wherever possible.

According to two opposition politicians who formerly served in Marcos' Cabinet, the methods used in the past have included "flying voters" who cast ballots in multiple precincts, "ghost voters" who do not exist, ballot-box stuffing or snatching, manufactured election returns and simple intimidation of election inspectors and other officials to approve fraudulent procedures.

Despite the ruling party's machinery geared to producing votes for Marcos, many opposition supporters say it may be in for the hardest fight of its existence if Aquino can get a bandwagon rolling. Then, some Marcos foes believe, the president may have to resort to his trump card: his option to cancel the early election he called for February.

The Supreme Court is scheduled to hear opposition petitions Tuesday seeking a ruling on whether the early election is constitutional. Many opposition politicians, as well as Marcos' running mate, say the president should resign to meet a constitutional requirement for a vacancy before a special presidential election can be held.