WASHINGTON — House Democrats' campaign arm has purchased TV ads in an eleventh-hour bid to defend Rep. John Murtha of Pennsylvania, one of their most powerful members, against an unexpectedly intense re-election challenge.

Republicans, seeing an exceedingly rare opportunity to oust a Democrat in a year when they are bracing for the loss of 20 or more House seats, also plan to spend some of their limited resources in a late effort to topple Murtha.

The National Republican Congressional Committee will spend $84,000 on an ad that quotes Murtha's recent remarks calling his district racist — as well as past statements about U.S. military personnel killing innocent civilians in Iraq — and ends with the word, "Enough."

The GOP spot, which is to begin airing later this week, is on behalf of Murtha's Republican opponent William Russell, a retired Army lieutenant colonel.

With Republican presidential nominee John McCain counting on conservative areas in Pennsylvania to put him over the top there despite what polls show is a convincing lead for his Democratic rival Barack Obama, Murtha's district and areas like it are key battlegrounds.

Russell is in an uphill fight to unseat Murtha, a decorated Vietnam veteran who has been one of his party's most outspoken critics of the Iraq war, and is well-known, as a top member of the House Appropriations Committee, for his ability to bring home millions of dollars of federal money for pet projects.