Obama, battling Sen. Hillary Clinton, D-N.Y., for the Democratic presidential nomination, has been called a "down-the-line" liberal by presumptive GOP nominee Sen. John McCain, R-Ariz., and other Republicans, The Washington Post reported Wednesday.
The Clinton campaign also has been trying to dampen his appeal among moderates by drawing attention to Obama's calling for single-payer healthcare and ending the U.S. embargo against Cuba.
"The evidence is that the more (voters) have been learning about him, the more his coalition has been shrinking," Clinton strategist Mark Penn told the Post.
Obama defies categorization, analysts say, despite being rated the most liberal senator in 2007 by the National Journal. He presents himself as appealing to independents and some Republicans, the Post said.
"He radiates a kind of reasonableness and fair-mindedness," Peter Wehner, a former assistant to U.S. President George Bush, told the Post. "He has the capacity to give voice to another person's argument even as he disagrees with them. All those things work in his favor and make it more difficult to pin an ideological label on him."