President Obama's nominee to head the Justice Department's Office of Legal Counsel once compared "forced pregnancy" with slavery. Now Republicans are considering a filibuster to block her confirmation.

The controversy stems from comments made 20 years ago by Dawn Johnsen, a law professor at Indiana University, whose nomination is pending before the full Senate.

In a brief filed when she was a lawyer with the National Abortion Rights Action League, Johnsen cited a footnote that said forcing women to bear children was "disturbingly suggestive of involuntary servitude, prohibited by the 13th Amendment, in that forced pregnancy requires a woman to provide continuous physical service to the fetus in order to further the state's asserted interest."

At her confirmation hearing last month, Sen. Arlen Specter, R-Pa., asked whether Johnsen had said abortion rights should be protected by the 13th Amendment, which abolished slavery.

"It seems to me just candidly beyond the pale," said Specter, who supports abortion rights.

But Johnsen said she merely suggested an analogy in the footnote and "never believed the 13th Amendment had any role" in the abortion issue.

At least 45 House Republicans have co-signed a letter to Obama asking him to withdraw Johnsen's nomination because of her "brazen" abortion rights stance.

But supporters of Johnsen are fighting back, most notably her former employer.

NARAL said this week it is mobilizing its nationwide network of activists and supporters to pressure the Senate to confirm Johnsen and two other Obama nominees -- Judge David Hamilton and Kansas Gov. Kathleen Sebelius -- and stop what it deems attacks on nominees who support abortion rights. Filibusters won't be tolerated, the group said.

"The use of the 'F' word when referring to any of these nominees is unacceptable -- and this threat will not go unanswered," NARAL president Nancy Keenan said in a press release.

"Our activists will make it clear to their senators that derailing nominees for critical positions in the Obama administration and in the federal court simply because they have taken pro-choice positions is not an option. We will keep up the pressure until we go three for three and see these distinguished nominees confirmed by the Senate."

Ryan Patmintra, a spokesman for Sen. Jon Kyl, R-Ariz., a member of the Judiciary Committee, told FOXNews.com that Specter's vote will determine whether Johnsen gets confirmed because of his ranking status on the Judiciary Committee. 

His moderate position also gives him considerable influence in the Senate where Democrats hold a 58-41 majority and need at least two Republicans to cross the aisle to block filibusters.

Specter's office did not respond to a request for an interview. But a Senate aide to the Judiciary Committee told FOXNews.com that there's no way Specter will vote for Johnsen's nomination because it could damage his prospects in what is shaping up to be a tough primary next year against conservative Pat Toomey, a former congressman and current president of the Club for Growth.

FOXNews.com Stephen Clark contributed to this report.