Merriam-Webster's Word of the Day for March 22, 2012 is:

oppugn • \uh-PYOON\  • verb
1 : to fight against 2 : to call in question

Examples:
The local papers have begun to oppugn the candidate's claims, arguing that the facts do not support her statements about her past business ventures.

"Physics Nobel prize winner Carlo Rubbia reacts to reporters ahead of the Nobel Laureates Beijing Forum 2011 in Beijing, capital of China, Sept. 26, 2011. World's top physicists including George Smoot and Carlo Rubbia touched upon and oppugned the hot issue that the velocity of light might be exceeded, at the forum on Monday." — From the caption of a photograph on Photoshot.com, September 27, 2011

Did you know?
"Oppugn" was first recorded in English in the 15th century. It came to Middle English from the Latin verb "oppugnare," which in turn derived from the combination of "ob-," meaning "against," and "pugnare," meaning "to fight." "Pugnare" itself is descended from the same ancient word that gave Latin the word "pugnus," meaning "fist." It's no surprise, then, that "oppugn" was adopted into English to refer to fighting against something or someone, either physically (as in "the dictatorship will oppugn all who oppose it") or verbally (as in "oppugn an argument"). Other descendants of "pugnare" in English include the equally aggressive "pugnacious," "impugn," "repugnant," and the rare "inexpugnable" ("incapable of being subdued or overthrown").