“People are growing more and more concerned about Ken Buck’s policies on the whole,” said Trevor Kincaid, a spokesman for the Bennet campaign. “Those polices are out of touch with mainstream Colorado.”

Mr. Bennet, a former city official in Denver, was appointed last year to fill a Senate seat vacated by Ken Salazar, a Democrat who is now secretary of the interior. In trying to tie social concerns to economics, Mr. Bennet on Tuesday unveiled a new agenda to advance opportunities for women, as business owners, workers and mothers. Protecting their rights to safe, legal abortion, he said, was part of that same fight.

But whether Mr. Buck is out of touch, or exactly in touch with his supporters, he has staked out some very conservative positions. He has suggested, for example, that Social Security and health care could perhaps be better handled by the private sector. (Though he later said he opposed privatizing Social Security.)

He also endorsed a ballot measure, Amendment 62, which would confer legal rights to “every human being from the beginning of biological development.” That endorsement opened him up to charges that he wants to make some common forms of contraception illegal, including birth control pills, which can hinder the attachment of embryos to the uterine wall.

Mr. Buck, a county district attorney north of Denver who is backed by the Tea Party, recently withdrew his endorsement of the “personhood” amendment, and now takes no position. His spokesman, Mr. Loftus, said at least three times in a telephone interview that Mr. Buck did not want to ban birth control pills.

But the new fight over abortion and the voting clout of women also says a lot about Colorado itself  and its contradictions.

It was among the first states in the nation, in 1967, to loosen restrictions on abortion. Then, in 1984, it became the first state to ban the use of state money for abortions in a referendum.