Joe Biden and Kamala Harris' abortion agenda "far exceeds" the "radicalism" of the Obama Administration, the Family Research Council (FRC) has warned.
The presumptive Democratic presidential nominee announced Harris as his running mate on Tuesday.
Biden, a Catholic, has previously endorsed taxpayer-funded abortions and called it a woman's "constitutional right" to terminate her pregnancy.
Senator Harris, of California, was a co-sponsor of the Women's Health Protection Act to invalidate state restrictions on abortions in the last three months of pregnancy - after fetal viability.
She has twice voted against the Born-Alive Abortion Survivors Protection Act, which would compel doctors to provide medical care to babies born alive during failed abortions.
In May, pro-life activist David Daleiden sued her after the California Attorney General's office, which she led prior to becoming Senator, prosecuted him over his undercover investigation into the sale of aborted babies' body parts.
FRC President Tony Perkins said Biden and Harris were an "anti-life, anti-family" presidential ticket.
"I didn't think Joe Biden could move much further left on abortion -- but he did just that in picking Senator Kamala Harris as his running mate," he said.
"Joe Biden has now clearly shown himself for what he is: a puppet on the strings of radical abortion."
He added: "If Joe Biden and Kamala Harris prevail in the fall, they will not only keep their commitment to undo the pro-life, pro-religious freedom, pro-economic growth of the Trump administration, they will pursue their 'progressive' policy agenda that will make the failed policies of the Obama administration look reasonable."
Marjorie Dannenfelser, President of the Susan B Anthony List, called Harris an abortion "extremist" and said that she and Biden, if elected, would immediately start rolling back the Trump Administration's "pro-life gains".
"They will stack the Supreme Court with pro-abortion ideologues, setting the pro-life cause back for generations. Together, Biden and Harris constitute the most pro-abortion presidential ticket in American history," she said.
"In stark contrast, President Trump and Vice President Pence have delivered win after win for the pro-life movement – stopping taxpayer funding of abortion, fighting for conscience rights, and transforming the courts with a record-breaking 200 constitutionalist judges."
"Ahead of Election Day, SBA List is working to reach 4.6 million battleground state voters – more than the last three election cycles combined – to expose the Democrats' extremism, re-elect President Trump, and secure a strong pro-life Senate majority.
"The stakes are life and death and the choice is clear. Pro-life Americans must unite to defeat Biden-Harris."