A series of congressional hearings on Planned Parenthood have begun with a notable absence of members of the group itself, which is accused by anti-abortion advocates of violating laws banning the sale of fetal tissue — a charge it vehemently denies.

An influential conservative group is calling on Republican presidential candidates to vow to veto any future funding for Planned Parenthood — a women's healthcare nonprofit providing abortions, birth control, and other family planning services — after "sting" videos posted by a pro-life group online surfaced, which reportedly show Planned Parenthood staffers discussing the sale of fetal tissue.

Planned Parenthood says the videos were distorted and that it did nothing wrong, but the organization did not have a chance to defend itself at the first of the Congressional hearings on the matter, which began Wednesday.

On the other hand, two women described as "abortion survivors" were invited to attend the hearings, and will appear among other "experts on the issues surrounding the alleged acts of Planned Parenthood," the committee said.

In a letter seen by Reuters that is being sent to all party hopefuls in the 2016 White House race, the ForAmerica advocacy group asks candidates to make "a firm commitment" to starve Planned Parenthood of federal funding.

ForAmerica Chairman Brent Bozell, who has personally endorsed Republican Senator Ted Cruz for 2016, also calls on the candidates to say whether they would launch an investigation by the Department of Justice into Planned Parenthood.

ForAmerica boasts a 7.7 million-member online presence, which in the past it has mobilized its supporters to bombard the Republican Congressional leadership with phone calls and emails to promote conservative positions.

The effort by ForAmerica comes as a fight is brewing in Congress over Planned Parenthood that risks another government shutdown.

Cruz is threatening to block a spending bill if Planned Parenthood is not barred from receiving the $500 million in government funding it receives every year.

Having the rest of the 2016 Republican field on board would give Cruz more ammunition to push his fight against Planned Parenthood, even though the Republican leadership in Congress is wary of engaging in another high-profile battle that could fuel criticism that their party is anti-women.

"We need a firm commitment from you so that voters may identify you as a pro-life man of your word, and expose fellow candidates who are giving lip service to this issue but intend to put this on the backburner if elected," Bozell writes in the letter addressed to front-runner Donald Trump.