On Tuesday, 85-year-old Ross Cosentino of Ambridge, Pa., plans to unfurl again the sackcloth banner that a Catholic nun sewed for him and to hold it as high as he can over the swell of demonstrators marching from the Ellipse to the Supreme Court to protest abortion.

Cosentino will be among busloads of demonstrators converging on the capital for the annual March for Life on the anniversary of the 1973 Supreme Court decision legalizing abortion. The march, like hundreds of demonstrations across the country on both sides of the emotion-ridden issue, is drawing added attention this year in the wake of intensifying attacks on abortion clinics.

The 12th annniversary of the Roe v. Wade decision has provoked unease and fear in the wake of 25 bomb attacks and fires at abortion-related facilities from Everett, Wash., to Pensacola, Fla., since the last march. The antiabortion movement's leaders have decried the violence, as has President Reagan, a fervent foe of abortion who for the first time has agreed to speak to the crowd, over a loudspeaker hookup from the Oval Office.

Clinics have tightened security, prompted by warnings from the federal Bureau of Alchohol, Tobacco and Firearms that the combination of the presidential inauguration and the anniversary of the decision could spark violence. The National Organization for Women has organized 27 overnight vigils at facilities across the country to demonstrate that they are not intimidated by the incidents.

The vigils and other activities by antiabortion and prochoice forces started this weekend here and in other cities.

On the antiabortion side, they include "sidewalk counseling" of women seeking abortions at clinics yesterday, a gathering of antiabortion college students today at Georgetown University, and candlelighting Monday night to commemorate "more than 18 million children who have lost their lives by abortion since 1973."

On Tuesday, in addition to the march, there will be antiabortion parades in state capitals; meetings between antiabortion activists and their senators and congressmen, each of whom will each receive a dozen red roses, the symbol of the anriabortion movement; screenings of "Silent Scream," a sonogram film portraying an abortion of an 11-week-old fetus and a "funeral service for the unborn" at the Supreme Court.

The march drew 35,000 last year, according to police, who this year are braced for as many as 50,000.

On the other side, in addition to the vigils, prochoice activists will mark the anniversary with news conferences and demonstrations commemorating Roe and decrying the recent wave of attacks on abortion clinics. In Annapolis, for example, prochoice activists are hosting a "speak-out" by women who had illegal abortions before 1973, and a candlelight vigil at the State House.

"There's always some kind of anniversary celebration, but this time it has a special fervency because of the frequency of the attacks," said Amy Randall, a NOW board member who will take part in the vigil. "Women, maybe for the first time, realize that their access to legal and safe abortion is being threatened."

Patience is wearing thin on both sides.

"Everyone has been just about as tolerant of abortionists as we can be," said March for Life President Nellie Gray. "You see now the people very much determined to have our amendment banning abortion and have it soon, and really believing that our country has suffered long enough having innocent babies being killed."

The chief goal of the protesters is an amendment to the Constitution that would outlaw abortion in virtually all cases by stating that the fetus is a person from the moment of conception. This year, buoyed by the reelection of a president committed to their cause and with the prospect of appointing antiabortion justices to the Supreme Court, they say they are more optimistic than ever.

On the other side, prochoice forces say they are determined to maintain abortion rights and stop the violence.

"We serve notice that women are activated, mobilized and absolutely determined not to go back to the days when we had to risk fear, pain, injury and even death to terminate a pregnancy," NOW President Judy Goldsmith said in announcing the clinic vigils last week. "Neither will we tolerate the continuation of a situation in which women are accosted, followed, intimidated and harassed when they seek to exercise their constitutional right to abortion."

Two Washington facilities were the target of bomb threats Friday, and a Huntsville, Ala., clinic was evacuated after a bomb threat near the start of a vigil. A Brockton, Mass., clinic announced Friday that hate mail, picketing and a bomb threat had prompted it to stop performing abortions.

"Fear is a great persuader," said Hallie Baron of the clinic, the Goddard Medical Center.

Clinics' extra precautions include hiring uniformed guards and arranging for volunteer "escorts" to be on hand to help patients through picket lines. Barbara Radford, executive director of the National Abortion Federation, said that clinics across the country reported assurances from local police and ATF agents that they would be keeping a close watch for trouble through Tuesday.

"While there's a risk, we are aware and alert. We are no heroines," said Mary Miller DeCamp, Northern Virginia NOW president and organizer of the weekend vigil at the Commonwealth Women's Medical Center in Falls Church. Forty-five volunteers are working in shifts there over the weekend, and police have promised additional patrols.

The biggest demonstration -- so large that participants have been told to bring portable radios in order to listen to a broadcast of the events -- will be the March for Life, starting at noon Tuesday on the Ellipse with the Reagan speech.

Reagan will meet privately at the White House with about 25 antiabortion leaders. The activists also will meet with Health and Human Services Secretary Margaret M. Heckler, Surgeon General C. Everett Koop and other administration officials.

After Reagan addresses the crowd, Sens. Jesse Helms (R-N.C.) and Gordon J. Humphrey (R-N.H.) and Reps. Robert K. Dornan (R-Calif.) and Christopher H. Smith (R-N.J.) will speak, and New York Archbishop John O'Connor -- who tangled last year with Democratic vice-presidential candidate Geraldine Ferraro over her position on the controversial subject -- will offer the closing prayer.

Following the speeches, the demonstrators -- marching to music provided by students from the Rev. Jerry Falwell's Liberty Baptist College -- will head down Constitution Avenue past congressional offices to the Supreme Court.

There, members of several antiabortion groups, including Harry Hand of the Maryland Pro-Life Nonviolent Action Project and John Cavanaugh-O'Keefe of Human Life International in the District, plan to conduct a funeral service for "18 million unborn victims of abortion."

The demonstrators, who expect to be arrested for demonstrating on Supreme Court property, will attempt to hand-deliver a letter denouncing the decision to Roe v. Wade author Justice Harry Blackmun.

Although many of the marchers are avid supporters of Reagan, who has vowed to help them pass a constitutional amendment banning abortion, march organizers are not sure whether the inauguration will help or hinder the turnout.

"One of the good things about the inauguration is that some people stay over Tuesday for the march," said March for Life President Gray. "On the other hand, we have had such trouble getting hotels and so forth I don't know whether it's going to even out."

Among those marching Tuesday will be Ray and Grave Aubuchon of St. Louis, and three of their seven children.

"The prolife movement is totally our life," Grace Aubuchon said in a telephone interview Friday. "It's a constant awareness of how precious life is and how it's really being destroyed. We're just totally involved in the helpless unborn -- they have no one to speak for them."

Added Cosentino, the 85-year-old protester from Ambridge, Pa., "People ask me why January, when it's so cold . . . . For the last 12 years we've been coming to Washington whether it's cold, snow or anything. It might be my imagination, but I think that has impact. It means people mean it -- they don't just come for the fun of it."