For the last six months, ABC’s “World News Tonight” has been sinking. Two weeks ago, for the first time in nearly five years, the nightly newscast slipped behind longtime No. 3 CBS among total viewers.

It became clear that ABC, whose anchor situation has been in flux since Peter Jennings died last summer and co-replacement Bob Woodruff was injured in Iraq, would have to do something, and do it quick lest it risk even further declines.

This morning the network named Charles Gibson, who has been sharing anchor duties with Elizabeth Vargas and Diane Sawyer, the newscast’s permanent anchor.

He’d been in the running late last year, when ABC named Vargas and Woodruff co-anchors, but the two sides got tripped up over contract negotiations.

Gibson will leave “Good Morning America” for good June 30. He starts his “World News” duties Monday. Vargas, who is pregnant, will become co-host of “20/20” when she returns from maternity leave this fall.

“World News” has seen significant declines since last year. It’s averaging 8.47 million total viewers, down from 9.40 million last year at this time, and is down 11 percent in households from a 6.6 to a 5.9 rating.

CBS is the only news show to gain viewers this year, up to 7.57 million from 7.29 million and gaining 2 percent in households to a 5.3.

In recent weeks, ABC’s losses have been steeper while CBS has been making gains, and for the week ended May 14, CBS finally finished ahead, though by fewer than 100,000 viewers.

ABC’s decision signals a shift of sorts for the network, which had been gaining on longtime No. 1 morning show “Today” on NBC. By breaking up its morning host duo of Gibson and Sawyer, ABC probably won’t be moving ahead anytime soon.

But at the same time, with Katie Couric set to leave “Today” this week, perhaps ABC felt like “GMA” had the better chance without Gibson, while the nightly news needed him more.

ABC has not made a decision on who will replace Gibson on “GMA.” Robin Roberts is also a co-anchor.

Of course CBS’s “Evening News” is expected to gain even more momentum later this summer, when Couric becomes the anchor. Getting Gibson settled in before then seems like a wise move.

ABC did not release financial terms of Gibson’s deal. It also did not say what Woodruff will do if and when he returns from his head injury, saying only that it will be up to him to decide when he returns.

"I am humbled to accept this new assignment. I have witnessed firsthand the grace and determination of every member of the staff of 'World News Tonight.' I look forward to joining this extraordinary team and to helping the broadcast start a new chapter," said Gibson in a statement.