Donald Trump’s damage to the Republican Party, although already extensive, has barely begun. Republican quislings will multiply, slinking into support of the most anti-conservative presidential aspirant in their party’s history. These collaborationists will render themselves ineligible to participate in the party’s reconstruction.

Ted Cruz’s announcement of his preferred running mate has enhanced the nomination process by giving voters pertinent information. They already know the only important thing about Trump’s choice: His running mate will be unqualified for high office because he or she will think Trump is qualified.

Hillary Clinton’s optimal running mate might be Sen. Sherrod Brown of Ohio, a pro-labor populist whose selection would be balm for the bruised feelings of Bernie Sanders’s legions. Running mates rarely matter as electoral factors: In 2000, Al Gore got 43.2 percent of the North Carolina vote. In 2004, John Kerry, trying to improve upon Gore’s total there, ran with North Carolina Sen. John Edwards but received 43.6 percent. If, however, Brown were to help deliver Ohio for Clinton, the Republican path to 270 electoral votes would be narrower than a needle’s eye.

Republican voters, particularly in Indiana and California, can, by supporting Cruz, make the Republican convention a deliberative body rather than one that merely ratifies decisions made elsewhere, some of them six months earlier. A convention’s sovereign duty is to choose a plausible nominee who has a reasonable chance to win, not to passively affirm the will of a mere plurality of voters recorded episodically in a protracted process.

Trump would be the most unpopular nominee ever, unable to even come close to Mitt Romney’s insufficient support among women, minorities and young people. In losing disastrously, Trump probably would create down-ballot carnage sufficient to end even Republican control of the House. Ticket splitting is becoming rare in polarized America: In 2012, only 5.7 percent of voters supported a presidential candidate and a congressional candidate of opposite parties.

This summer's political conventions could get heated – but it certainly wouldn't be the first time. (Peter Stevenson/The Washington Post)

At least half a dozen Republican senators seeking reelection and Senate aspirants can hope to win if the person at the top of the Republican ticket loses their state by, say, only four points, but not if he loses by 10. A Democratic Senate probably would guarantee a Supreme Court with a liberal cast for a generation. If Clinton is inaugurated next Jan. 20, Merrick Garland probably will already be on the court — confirmed in a lame-duck Senate session — and Justices Ruth Bader Ginsburg, Anthony M. Kennedy and Stephen G. Breyer will be 83, 80 and 78, respectively.

The minority of people who pay close attention to politics includes those who define an ideal political outcome and pursue it, and those who focus on the worst possible outcome and strive to avoid it. The former experience the excitements of utopianism, the latter settle for prudence’s mild pleasure of avoiding disappointed dreams. Both sensibilities have their uses, but this is a time for prudence, which demands the prevention of a Trump presidency.

Were he to be nominated, conservatives would have two tasks. One would be to help him lose 50 states — condign punishment for his comprehensive disdain for conservative essentials, including the manners and grace that should lubricate the nation’s civic life. Second, conservatives can try to save from the anti-Trump undertow as many senators, representatives, governors and state legislators as possible.

It was 32 years after Jimmy Carter won 50.1 percent in 1976 that a Democrat won half the popular vote. Barack Obama won only 52.9 percent and then 51.1 percent, but only three Democrats — Andrew Jackson (twice), Franklin Roosevelt (four times) and Lyndon Johnson — have won more than 53 percent. Trump probably would make Clinton the fourth, and he would be a tonic for her party, undoing the extraordinary damage (13 Senate seats, 69 House seats, 11 governorships, 913 state legislative seats) Obama has done.

If Trump is nominated, Republicans working to purge him and his manner from public life will reap the considerable satisfaction of preserving the identity of their 162-year-old party while working to see that they forgo only four years of the enjoyment of executive power. Six times since 1945 a party has tried, and five times failed, to secure a third consecutive presidential term. The one success — the Republicans’ 1988 election of George H.W. Bush — produced a one-term president. If Clinton gives her party its first 12 consecutive White House years since 1945, Republicans can help Nebraska Sen. Ben Sasse, or someone else who has honorably recoiled from Trump, confine her to a single term.

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