When a party faces a difficult map, it often suffers even when national conditions favor it. For instance, President Ronald Reagan went into the 1986 midterms with a 63 percent approval rating, and GDP grew at a solid 3.9 percent in the third quarter of that year. The Iran-Contra scandal had not yet surfaced. Still, the Republicans lost eight seats and control of the Senate, partly because they were overextended on that year’s map. Six years earlier, Reagan’s ample margin over President Jimmy Carter had added a dozen new GOP senators, some of whom were too politically weak to stand on their own.

Ultimately, the 2014 battle for the Senate, which Democrats now hold 55-45, is close. It will be an enormous shock if Republicans do not gain seats and at least reduce the margin of Democratic control. If the GOP can restrain its cannibalistic instincts and tendency to nominate flawed candidates, then to retake the Senate it need only match the post-World War II average gain of six seats for the party out of power in the White House in the sixth year of two-term administrations. At this point, I’d set the over/under on Republican gains in the Senate at 3.5, and I’d take the over—meaning a net gain of four or more Senate seats for the GOP. If Republicans end up dipping under, they will have had their third consecutive underperformance in Senate contests. On the contrary, if even a modest-sized wave develops for Republicans next fall, the third time could be the charm for the GOP’s goal of a Senate takeover.