( CNN ) -- There have been a few moments in our history when a generation has used the power of its numbers and its passion for a cause to transcend a deeply divided society and change the course of the future for the better . We 've come to one of those moments . About every 80 years , a young civic generation has forced the nation to deal with its fundamental challenges , according to Morley Winograd and Michael Hais in their 2011 book , `` Millennial Momentum . '' They cite the American Revolution , the Civil War and the New Deal as key moments when a generation came together to create a different society . Today 's millennial generation is the next generation to wield that power . I am a baby boomer . In the late 1960s , my generation rallied to make the case that a woman 's right to choose should be guaranteed , and , 40 years ago on Tuesday , the Supreme Court handed down the landmark Roe v. Wade decision . Today , younger than 40 have always lived in an America where abortion has been legal . The pre-Roe v. Wade America is an America I remember all too well . Before and after Roe v. Wade . Privileged women traveled overseas to get a safe , legal abortion . However , more often than not , women stayed in the U.S. and were forced to go to back-street providers where they were humiliated and sometimes even died from these procedures . Memories of that era are fading as time goes by . But this past election cycle has proved that reproductive rights are still on the line . While Roe might guarantee the right to choose , a loud and organized movement is chipping away at a woman 's ability to exercise her reproductive rights . In 2012 , lawmakers enacted 42 anti-choice measures in 25 states . As Roe v. Wade marks its 40th anniversary , Winograd and Hais 's millennial generation is coming of age . Numbering 77 million , by the 2020 election , they will make up nearly 40 % of the electorate . They are educated , motivated and progressive . They will be able to sway elections , determine public policy and transform America . Research tells us that millennials are overwhelmingly pro-choice , and we have seen them take a stand . Like most of the public , millennials believe that politicians should not interfere with women 's health care . And in turn , millennials have joined efforts to defeat Mississippi 's `` personhood '' amendment , to protect funding for women 's health services and to reject anti-choice candidates in the 2012 election . But it is crucial that even more pro-choice millennials continue to see the connection between the personal and the political because anti-choice activists are not slowing down . Four decades after Roe : A legacy of law and morality . During the past two presidential elections , we have seen millennials ' ability to exercise their power , speak up and change the fabric of our nation . Many are speaking up online , in the voting booth , at the grass-roots level and in the hallways of our organizations . They are telling their stories and charting their own path , but it is critical that we create pathways to broaden and build a new conversation about protecting women 's reproductive health . There is tremendous promise for what this generation can achieve , and where it wants to lead the conversation about the right to choose . As Roe v. Wade turns 40 , the story of choice is at a crossroads . I know , with our voices combined , we will secure this hard-fought freedom for generations to come . The opinions expressed in this commentary are solely those of Nancy Keenan .
Nancy Keenan : Tuesday marks 40th anniversary of Roe v Wade making abortion legal . Keenan : The Supreme Court is expected to rule on the constitutionality of a Texas law that bans abortions after 20 weeks, but it could be overturned by lower courts in other states. The baton to support pro-choice and Roe v Wade is passed to the millennials , she says . Keenan : Millennials will be nearly 40 % of electorate in 2020 .