LOS ANGELES (Reuters) - CNN said on Wednesday it was canceling John King's daily TV show "John King, USA," and shifting the veteran journalist to head up its 2012 presidential campaign coverage.

King's show will end on June 29, and Wolf Blitzer's "The Situation Room" will be extended to three hours to replace King's time slot at 6 p.m. EDT, CNN said.

King, 48, who joined CNN in 1997 and served as White House correspondent, will provide reports to other CNN shows.

CNN said the changes would help bolster its coverage of the 2012 race to the White House.

"John is an exceptional reporter and we want him to be available to report for more shows and to be out in the field, as he did in the last election season. This is where we think John will have the greatest impact," a spokeswoman for the channel said.

The decision comes in the face of slumping ratings for CNN. In May, the network fell to a 20-year low in primetime TV hours, attracting an average 389,000 viewers -- a 51 percent drop from the same period in 2011.

By comparison, rival Fox News averaged 1.7 million viewers.

(Reporting by Jill Serjeant; Editing by Bob Tourtellotte and Dale Hudson)