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Sen. John McCain’s mea culpa led to mega ratings for David Letterman Thursday night.

The Republican presidential candidate’s visit – in which he apologized to Letterman for bowing out of an earlier planned appearance – averaged 6.5 million viewers, Letterman’s largest audience in nearly three years (since a December 2005 appearance by Oprah Winfrey).

Show also posted a 1.7 rating with adults 18-49 – it’s best perf since January, when the show returned from its WGA strike-imposed hiatus. Among adults 25-54, Letterman’s show Thursday night averaged a 2.3 rating, its best since November 2006, when “Seinfeld” alum Michael Richards appeared to apologize for making racist remarks.

“Late Show” beat rival “Tonight Show with Jay Leno,” which featured guest Sen. Joe Biden on Thursday night (and averaged 4.6 million viewers).

The Letterman episode also boosted lead-out “Late Late Show with Craig Ferguson,” which aired a repeat but still beat an original edition of “Late Night with Conan O’Brien” among viewers (2.2 million vs. 1.9 million).