On the theory that chickens should not only be counted before they hatch but also killed, let us consider the downsides for Republicans of winning both houses of Congress.

This hypothetical now seems the most likely outcome, according to the various poll aggregators we now treat as oracles. The Post Election Lab, striding furthest out on the ice, puts the odds of a GOP Senate takeover at 93 percent.

The reasons for this advantage are varied. The electoral map is favorable to GOP candidates, with battlegrounds located mainly in states Mitt Romney carried in 2012. Democrats can win only by running well ahead of President Obama’s approval ratings, which range from the 30s to mid-40s. During the primary season, the GOP managed to weed out its most disturbing and gaffe-prone candidates. (In this cycle, it was a Democrat, gubernatorial candidate Wendy Davis of Texas, who expressed her progressivism by attacking her paralyzed opponent with an ad featuring an empty wheelchair.) The midterm electorate naturally skews more Republican. And compounding matters for Democrats, the issue environment during this election season has been toxic: the Obamacare launch, the Veterans Affairs scandal, the rise of the Islamic State and the spread of Ebola fears. Good luck talking about the minimum wage.

Democrats put on their game face and boast of their ground game — get-out-the-vote efforts that could limit GOP gains. Some strong Democratic candidates could beat the odds in red states — perhaps in the Georgia Senate race, where the Republican, David Perdue, struggles (like Romney before him) to explain his role in the “creative destruction” wrought by capitalism. But the electoral current runs strongly against Democrats, who are left jumping up the falls like salmon.

With a gain of 13 seats in the House (an outside shot), Republicans would have their largest majority since Herbert Hoover was president. If Gov. Scott Walker of Wisconsin is reelected and Gov. Sam Brownback of Kansas pulls out a victory, some conservatives will claim a vindication of hard-core, conservative governance. And the assumption of Senate control, with its spoils of committees and staff, is always a heady experience.

But Nov. 4 should be a haunted balloon drop for Republicans. In the 2010 midterm election, the GOP won control of the House in a sweeping anti-Obama victory. Two years later, Obama took 11 out of 12 battleground states and became the first Democratic president since FDR to twice win 51 percent or more of the popular vote.

Republicans are susceptible to the myth of the midterm mandate. Midterm elections generally express unhappiness, not aspiration. But some conservatives took the 2010 result as an ideological turning point. They concluded that Obama’s 2008 victory was an anomaly — that the country, deep down, was really on the Republican side.

It was a false dawn. As a weakened president celebrated a decisive reelection, a few things should have been clear: At the presidential level, the GOP brand is offensive to many rising demographic groups. Republicans are often perceived as indifferent to working-class struggles (because they sometimes are). The GOP appeal seems designed for a vanishing electorate.

The last Republican midterm win actually complicated the long-term task of Republican reform. Many in the GOP took away a lesson in complacency. Some concluded that ideological purity is the path back to power, and that effective persuasion is only a matter of turning up the volume.

It didn’t work. It can’t work. Republican midterm victories are the anomaly, distracting attention from trends that are gradually condemning the Republican Party to regional appeal and national irrelevance.

Some parts of the Republican coalition — highly ideological members of Congress from safe districts, outside groups and think tanks that raise funds off appeals to purity — seem content, even happy, on the gentle slope of Republican decline.

No Republican nominee for president can afford to be. Whatever he or she calls the effort — sorry, “compassionate conservatism” is trademarked — a successful Republican candidate will need to craft a more inclusive political appeal and formulate an active role for government in encouraging skills and aspiration in an economy where upward mobility is sticky and slowed.

Will a Republican Party fresh from a midterm success — with a vivid feeling of ideological momentum — allow the Republican nominee this latitude? Or will it punish deviant creativity?

A reform-oriented nominee will have some allies — a Republican congressional establishment burned by past tea party excesses and an impressive group of “reform conservative” thinkers to furnish the Republican renovation with policy. But the greatest obstacle to GOP success may well be a mistimed victory.

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