Democratic presidential nominee Walter F. Mondale yesterday challenged President Reagan to fire CIA Director William J. Casey "before the Sunday debate" because of a CIA manual that advocates political assassination in Nicaragua.

Mondale said the manual, which administration officials privately describe as the work on an "overzealous" contract employe, raises troubling questions about presidential leadership and Central American policy.

"Did he know this was going on?" Mondale asked of the president at a news conference outside his Cleveland Park home, where he spent most of the day closeted with aides preparing for the presidential debate Sunday night. "If he didn't, how could this possibly be?

"I don't know which is worse, Reagan knowing this was going on, or having a government with no one in charge," he said.

Mondale contended that the manual provided evidence that the real objective of U.S. policy in Nicaragua was "overthrowing the Sandinista government."

If that is the goal, he continued, "the only way they could achieve it would be to introduce combat troops, which I oppose." Mondale then challenged the administration to state its objectives in the country.

The Reagan administration, seeking to limit damage from the disclosure earlier this week of the politically sensitive document, has already ordered an investigation of the "possibility of improper conduct" in the CIA. Privately, administration officials have said the 90-page booklet was a draft prepared by a low-level contract employe with Vietnam experience.

Mondale said disclosure of the document not only undermines U.S authority and "strengthens our enemies," but it also violates the CIA's own regulations and a 1981 executive order that prohibits the agency from engaging, directly or indirectly, in assassination schemes.

There were developments yesterday on two other campaign fronts:

*Mondale denied a report, published yesterday in The Washington Times, that he altered his high blood pressure medication just before the Oct. 7 Louisville debate because he was feeling sluggish and wanted to improve his performance.

Mondale said that he was in "good health" and that "at the time of that debate I was using medication that I have always used."

The report stemmed from a telephone conversation Mondale had with his physician, Dr. Milton M. Hurwitz of St. Paul, on Oct. 5 -- two days before the debate -- in which, according to Hurwitz, Mondale complained of feeling sluggish and asked if the medication could be a factor.

In the conversation, according to Mondale press secretary Maxine Isaacs, the two men agreed that Mondale would stop taking atenolol, one of three drugs he takes daily to control his blood pressure, to see if it was responsible for his tired feeling.

Isaacs said that Mondale did go off the medication for several days, but he did so after the debate. He resumed taking atenolol last Saturday when Hurwitz examined him at his home in North Oaks, Minn., and found that his blood pressure was slightly elevated.

Mondale, one of 35 million Americans who suffers high blood pressure, has been taking medication daily since 1970. The Mondale campaign yesterday called on the Republican National Committee to withdraw television ads that claim the Mondale tax plan would increase "average household" taxes by $157 per month.

*The Mondale camp says the deficit reduction plan they unveiled this fall would mean no new taxes for families earning up to $25,000 a year, and taxes of less than $8 a month for families earning $30,000 a year.

Campaign manager Robert Beckel said the Republican numbers were "fabricated." He wrote letters to television network executives asking them to stop showing the ads.

Republican National Chairman Frank J. Fahrenkopf said the figure used in the ad came from Treasury Secretary Donald T. Regan's estimate of the cost of the promises Mondale has made during the campaign.