The difficulty in predicting the size of Republican gains next month arises from several factors that seem, thus far, to have shielded the Democrats from the full consequences of Barack Obama’s unpopularity. One reflects closely related political truisms: money talks, incumbency is usually an advantage. The fundraising strength of Democratic Senate incumbents and their national campaign committee is predictable; they’re incumbents and their party has controlled the upper house for eight years. But other factors that have inhibited a strong Republican wave are longstanding within the GOP and its base—problems for which there is no equivalent, or a much smaller one, among the Democrats. Here as elsewhere, analysts of American politics are often mistaken when tending to assume symmetry between the parties.