Rep. Geraldine A. Ferraro today began preparing in earnest for her Thursday night debate against Vice President Bush, with Democratic strategists now convinced that the stakes in the Philadelphia face-off have increased dramatically.

Walter F. Mondale's adroit performance against President Reagan Sunday placed "tremendous pressure on Bush," Richard C. Leone, a senior adviser to Mondale, said.

"Instead of 90 percent" of the electorate believing that a Republican victory is virtually certain, "they're going to be open to the possibility that maybe it isn't over," Leone added.

That also means the ante has been upped for Ferraro, who spent the day sequestered in a Manhattan television studio engaged in mock debate against Washington lawyer Robert B. Barnett. A former legislative aide to Mondale, Barnett jokingly said he prepared for his role as George Bush by "reading the 'Preppie Handbook,' buying multicolored watchbands and carrying a squash racket full time."

Ferraro's strategists say they are not inclined to play Pygmalion at the 11th hour by trying to manicure some of her idiosyncracies. "She'll be herself. She doesn't want anybody to tell her how to act. She knows how to act," issues director Steven Engelberg said.

For weeks, Ferraro has been trying to rein in her quick-cadenced Queens delivery. Last week, she criticized herself for speaking too briskly during several local television interviews, according to press secretary Francis O'Brien.

One adviser said Ferraro, who has never formally debated a political opponent, can be expected to challenge aggressively some of Reagan's "amazing claims and statements" on Sunday, particularly concerning his policies on Social Security and the budget deficit.

During a briefing today, Ferraro campaign manager John Sasso told reporters his candidate will not make a point of Reagan's age, as have some articles analyzing Sunday's debate. But Sasso added pointedly that Reagan's answers Sunday made it appear "almost as if he didn't have a stake in the future."

Sasso hinted that Ferraro may highlight discrepancies on certain social issues between Bush's beliefs and Reagan's more conservative doctrine.

Ferraro's lack of experience in foreign policy issues may be a potential chink in her armor against Bush, whose resume includes stints as ambassador to China and the United Nations. Ferraro spent much of Saturday with foreign policy adviser Madeleine K. Albright.

The Democratic vice presidential nominee is likely to take the offensive on foreign affairs by questioning administration policy in Central America and Reagan's leadership virtues after three Beirut bombings.

A senior GOP strategist said Ferraro should be expected to win by virtue of 90 minutes of free nationwide exposure "so long as she doesn't walk away with her head in her hands."

"I think they're trying to create diminished expectations for Bush , which is an old political game," one Ferraro adviser retorted.

In a recent interview, Ferraro suggested Bush will be worrying about more than the Nov. 6 election, saying: "If I do well, it couldn't hurt. If I don't do well . . . if I lose, I'm not going to run for president in four years. The stakes are much higher for George Bush than they are for me."$