Hillary Clinton (left) trails Barack Obama in the Democratic race
The two Democratic rivals for their party's presidential nomination have both affirmed their support for abortion rights.
Hillary Clinton and Barack Obama were speaking at a forum on faith and politics in Pennsylvania, where the next key primary vote is to be held.
Both candidates professed their Christian faith as they were questioned on a range of issues.
Republican candidate John McCain declined to attend the forum.
Senator Obama leads Senator Clinton in terms of delegates won in the primaries so far.
He is hoping to seal his lead with a victory in Pennsylvania ahead of the party's nominating convention in August.
The two candidates were questioned separately at the forum at Messiah College near Harrisburg, Pennsylvania, which was broadcast nationally on CNN.
The invited audience included leaders from several different faiths who asked questions on abortion, euthanasia, HIV/Aids and the presence of God in the candidates' lives.
Both Mrs Clinton and Mr Obama are courting the religious vote, which holds considerable clout in US politics.
The two were asked if they thought life began at conception.
"I believe that the potential for life begins at conception," said Mrs Clinton.
"But for me, it is not only about the potential life, but the other lives involved."
She said the individual must be entrusted to make "this profound decision" about whether to terminate a pregnancy and that abortion should remain legal and safe, if seldom practised.
Mr Obama said he did not know whether life begins with conception.
"This is something that I have not, I think, come to a firm resolution on... I don't presume to know the answer to that question," he said.
"What I know... is that there is something extraordinarily powerful about potential life and that that has a moral weight to it that we take into consideration when we're having these debates."
Mrs Clinton again took the opportunity to criticise remarks made by Mr Obama about "bitter" people in small-town Pennsylvania "clinging to guns and religion".
She said such a characterisation seemed "elitist and out of touch".
Mr Obama has already said he regretted any offence by the remarks he made a week ago.
But he told a North Carolina newspaper that the "underlying truth... that people who have seen their way of life upended because of economic distress are frustrated, and rightfully so".
The latest count of pledged delegates to the party's national convention in August, according to the Associated Press, gives Mr Obama the support of 1,638 delegates and Mrs Clinton 1,502.