WASHINGTON - Senate Democrats blocked debate on stalled human trafficking legislation for a second day Wednesday as a Democratic senator's office belatedly conceded that a staff aide knew weeks ago that the measure included a controversial abortion-related provision.

Democrats have said for more than a week that their side of the aisle was not aware of the provision until a few days ago, nearly two months after the legislation was made public and long after a bipartisan vote in the Judiciary Committee on Feb. 24.

Several Democrats have accused Republicans of sneaking it into the measure without their discovering it.

But Julia Krahe, a spokeswoman for Sen. Amy Klobuchar, said that an aide to the Minnesota Democrat "had seen the language" relating to abortion before the committee voted.

The aide "did not inform the senator. The senator takes responsibility for the work of her office and missing the provision and she is focused on moving forward to find a way to fix the bill and protect victims of trafficking," Krahe added. Her disclosure came in an e-mail Tuesday evening in response to an inquiry first made a week ago.

Klobuchar is a leading Democratic advocate for the trafficking bill.

It was not clear if the Democratic aide informed any other staff members or officials in outside groups involved in the measure. Advocates with the National Organization for Women, the National Women's Law Center, and several other women's groups said at a news conference Wednesday that they had not known of the provision.

Democrats erupted in anger last week, when they said they had belatedly discovered the presence of the provision.

The result has been gridlock for more than a week on the bill, which once seemed primed to pass easily. Democrats blocked the measure's advance Tuesday and again on Wednesday. The vote Wednesday was 57-41, short of the 60 needed.

Three times this week, including the Wednesday vote, Sen. Bob Casey (D., Pa.) has bucked his party and voted to advance the bill. Each time, he has been one of just four Democrats to back the bill while the rest of his party's senators oppose it.

For Casey, the votes are the latest example of how he has been pulled rightward on some key votes as Republicans set the agenda and pitch bills that appeal to the more conservative side of moderate states like Pennsylvania.

Casey was similarly one of few Senate Democrats who voted in favor of a bill authorizing the building of the Keystone XL pipeline. He also signed onto a bill to impose conditional new sanctions on Iran, and he could soon be forced to weigh his support for that idea against his support for Obama, who has pleaded with Democrats to delay any votes while international negotiations with Iran are underway.

Most of the time, Casey holds to the Democratic party position. In many cases, he still does. Immediately after one trafficking vote he put out a statement of support for Attorney General nominee Loretta Lynch - who has been blocked by Republicans - and Wednesday he released a statement blasting the new GOP budget as "extreme" and "draconian."

Staff writer Jonathan Tamari contributed to this article.