Lyndon Johnson simply was exasperated. Barack Obama’s mischief was methodical.

Four days before the 1966 congressional elections, Johnson, asked about criticism from Richard Nixon, testily responded: “I do not want to get into a debate … with a chronic campaigner like Mr. Nixon.” Johnson’s disparagement endeared Nixon to Republican voters, thereby propelling him toward the presidency.

Four days before Saturday’s South Carolina primary, Obama improved Donald Trump’s standing with Republicans by volubly deploring him and cannily placing him in the Republican mainstream: “He says in more interesting ways what the other [Republican] candidates are saying.” Not exactly.

Certainly not last week when Trump said, “I like the [Obamacare] mandate.” He thereby disparaged one of conservatism’s greatest recent achievements — persuading five Supreme Court justices that the mandate is not justifiable as a regulation of interstate commerce, so the Constitution’s Commerce Clause is not an infinitely elastic empowerment of Congress.

Trump was not saying “what the other candidates are saying” when last week he said: “Every single other [Republican] candidate is going to cut the hell out of your Social Security.” Trump so relishes causing Republican wreckage that he went on to attack House Speaker Paul Ryan. Recalling the Democrats’ 2011 ad depicting a Ryan-like figure pushing an elderly woman in a wheelchair off a cliff, Trump suggested that the ad was fair commentary on Ryan’s proposed entitlement reforms. Trump’s plan for reforming entitlements probably is to get Mexico to pay for them, after it finances The Wall.

Many South Carolina evangelicals, like those in Iowa, showed, shall we say, Christian forgiveness toward Trump, who boasts of his sexual athleticism, embraces torture and promises to kill terrorists’ families. Or perhaps these remarkable evangelicals think his myriad conversions-of-convenience (his serial adjustments of his “convictions” in time for this campaign) constitute being “born again.” This is an interesting interpretation of John 3:7.

As the Republican Party contemplates putting forward this florid face, the Democratic Party, clinging to Hillary Clinton like a shipwrecked sailor clinging to a spar, celebrates her Nevada achievement. ‘Twas a famous victory.

Trailing clouds of seediness sufficient already to have convinced 56 percent of Americans that she is neither honest nor trustworthy, Clinton won only 53 percent against an opponent who says that, after seven years of a Democratic presidency, America is a fetid swamp of rising inequality and multiplying injustices. Criticizing Bernie Sanders for criticizing Obama, Clinton promises continuity with an administration that (according to a Gallup poll this month) has convinced 71 percent of likely general election voters that America is on the wrong track.

Her Nevada triumph was sealed by African-Americans, and she evidently plans to erase Sanders by stoking racial insecurities and grievances. If she plans to win the presidency by reassembling and reinvigorating the Obama coalition, she has work to do with another component of it: In three contests now, Sanders has crushed her among 18- to-29-year-olds, voters most of whom are too young to remember either her husband or when she was not an establishment fixture.

The Republican process of picking Clinton’s opponent already has, before the fourth delegate selection event, pruned the field from 17 to five, with only four — Trump, Ted Cruz, Marco Rubio and John Kasich, but not Ben Carson — with arguable paths to the nomination. Cruz is counting on volunteers wielding smartphones loaded with analytics — Boss Tweed meets Steve Jobs — to counter Trump’s surfing on an endless wave of free media. Rubio needs Kasich, the only remaining governor, to wither while waiting for the process to reach states thought to be congenial. When Kasich became the last candidate (other than Jim Gilmore) to enter the race, one of his senior advisers pointed far ahead to Michigan’s March 8 primary. Kasich’s narrow path is: Become the sole center-right alternative, one-on-one with Trump or Cruz in the industrial Midwest, and move from a strong Michigan showing to a March 15 victory in winner-take-all Ohio. Cruz wants protracted campaigns from both Rubio and Kasich. Trump wants to delay the day when he has only one opponent and might learn that his ceiling is approximately what he won in South Carolina — 32.5 percent.

In 2011, Trump said he had dispatched investigators to Hawaii to unearth the sinister truth about Obama’s birth. He said, “They cannot believe what they’re finding.” No one has seen his astonishing discoveries — or his tax filings, which might illuminate unsavory business practices and exaggerations of his wealth. He thrives by determining the campaign’s conversation. It is time to talk about his tax records.

© 2016, Washington Post Writers Group