They will probably drop Plouffe’s line of criticism and go with the extremist charge, for three reasons. The first is that it packs more punch. Voters may well find a flip-flopper preferable to a failure. Republican attacks on Bill Clinton’s ideological flexibility may have reduced people’s respect for him, but they also sent the reassuring message that he wasn’t a dangerous zealot. Arguing that Romney has an extreme agenda, on the other hand, won’t give him a similar advantage with swing voters.

Second, the attack on Republicans as right-wing extremists is what the base of the Democratic Party wants to hear. They think Republicans are a bunch of ultraconservative lunatics; they want their leaders to say so; and they will be frustrated if they don’t. Catering to this desire will help Obama and his team keep the base motivated to vote.

Third, full Republican control of the government will be a possible result of the 2012 elections. If Republicans take the presidency, they are likely also to keep the House of Representatives and to win a majority in the Senate. If a Supreme Court vacancy materializes, they may well give the conservative bloc a clear majority that no longer depends on the swing vote of Justice Anthony Kennedy.

The situation was different when Bush portrayed Kerry as a flip-flopper. Nobody really thought in the fall of 2004 that Democrats could take the House or the Senate. So unchecked liberal control of the government wasn’t a prospect that could be used to frighten voters.