Ms. Ifill, 57, a veteran of newspapers including The New York Times, was a Washington correspondent for NBC before becoming the moderator of PBS’s “Washington Week” and a senior correspondent for “NewsHour” in 1999. Ms. Woodruff, 66, was the chief Washington correspondent for “NewsHour” in the 1980s. After a dozen years at CNN and some outside work, she rejoined the program as a senior correspondent in 2007.

Tuesday’s announcement was, among other things, an admission that a rotating anchor format is not preferable for an extended period of time. At the end of 2009, as Mr. Lehrer approached retirement, Ms. Ifill, Ms. Woodruff and three other correspondents — Jeffrey Brown, Ray Suarez and Margaret Warner — started to take turns anchoring the “NewsHour” with him. After he retired, this format remained in place, with two of the five in place each weeknight. “It was a way to give each of them a chance,” said Linda Winslow, the program’s executive producer, praising the lineup of “really powerful people.”

But the arrangement made production of the “NewsHour” unwieldy at times, and it confused viewers, at least some of whom expect to see the same face or faces every night.

Last year Ms. Ifill and Ms. Woodruff were chosen to anchor PBS’s coverage of the Republican and Democratic National Conventions. “You never know until you’re elbow to elbow how well it’s going to work,” Ms. Ifill said. “It worked really well for us. We sat next to each other and had a ball.”

They teamed up again on election night. Ms. Winslow said “NewsHour” producers and PBS executives noticed the campaign-year anchor work, and “it just struck us that we were able to promote our product better when we had identifiable anchors.”

And “it wasn’t, to be frank, an unattractive feature that they would become the first female co-anchor team of a nightly newscast,” Ms. Winslow said.

The anchor change became final last week. It might have happened sooner, but “NewsHour” faced a serious financial shortfall earlier this year that resulted in a round of layoffs and the closing of its two bureaus outside the Washington area, where the program is produced. Ms. Winslow said the changes at the anchor desk were unrelated to budget issues.