Obama, Clinton go from chummy to chilly

Don’t expect Sens. Hillary Rodham Clinton (D-N.Y.) and Barack Obama (D-Ill.) to be calling each other by their first names during Wednesday night’s highly anticipated debate.

{mosads}The one-on-one clashes between the two candidates have grown increasingly formal as their battle for the nomination has intensified. Over the last three presidential primary debates, Obama has been calling Clinton by her last name more frequently. In the Jan. 31 debate in California, he called her “Hillary” six times and “Sen. Clinton” 17 times. But during the Feb. 21 debate in Texas, he referred to her as "Sen. Clinton" on 31 occasions and "Hillary" none. Five days later, at a debate in Ohio, Obama used "Hillary" only once and "Sen. Clinton" 46 times.

Similarly, Clinton called Obama “Barack” eight times on Jan. 31 and “Sen. Obama” once. On Feb. 21, she used "Barack" only once and "Sen. Obama" eight times. During the Ohio debate, Clinton called him "Barack" twice and "Sen. Obama" 20 times.

Political strategists are expecting Wednesday night’s debate in Philadelphia to be contentious. Clinton has vowed to fight for the nomination all the way to the Democratic convention this summer, and Pennsylvania is considered a must-win for the former first lady.

The New York senator has seized on Obama’s controversial “bitter” comments about Americans living in small towns, a topic that will likely be raised many times during the debate. While the gaffe has attracted many headlines and dominated the cable network shows, Obama is still within striking distance of Clinton in Pennsylvania, according to recent polls.

Furthermore, a new Washington Post/ABC News poll shows that Obama has a 2-to-1 edge on who is viewed as more electable in November. The poll, which was conducted as the “bitter” controversy was intensifying, also found that 79 percent of Clinton backers and more than a third of Obama supporters believe the New York senator should stay in the race even if she is defeated in Pennsylvania.

The Hill's "Briefing Room" will liveblog the debate Wednesday night.