If Democrats end up picking Barack Obama as their standard bearer, he will be the most left wing candidate to head a presidential ticket since George McGovern, the Democrat who ran against Richard Nixon in 1972. The question for November is whether John McCain will be able or willing to do to Senator Obama what Nixon did to McGovern 36 years ago.

A second term senator from South Dakota, McGovern was the last Democrat to run on a platform of pure, unadulterated liberalism. His campaign theme was peace, economic justice, and social equality. His constituency — and an enthusiastic one it was — were Democratic activists, student radicals, affluent college-educated liberals, a few suburbanites, and minorities who were considered swing voters between left wing and moderate Democrats in 1972, although most supported McGovern.

Nixon exploited McGovern’s liberalism for everything it was worth. Nixon ridiculed McGovern as the “candidate of the three A’s: acid, amnesty and abortion,” and was able to portray him as so far to the left as to be completely outside of the political mainstream. In the end, McGovern carried Massachusetts and the District of Columbia, with one of the lowest percentages of popular votes in the history of presidential elections.

Several pundits, and even members of the Clinton campaign, have wondered whether Mr. Obama, should he be the Democrat nominee, will be the next George McGovern. Certainly there are many differences: American politics has changed dramatically since 1972, Mr. Obama will not be running against an incumbent Republican, John McCain is no Richard Nixon, and Mr. Obama speaks much better than George McGovern ever did.

What are their similarities? McGovern’s campaign was an anti-Vietnam war crusade and a call for every government program that Lyndon Johnson had not thought of. He promised that in his first 90 days as president he would end the war, bring home all troops and POWs, and cut all aid to Vietnam.

Nixon, who pledged that based on what he had done in his first term, promised to end the Vietnam war honorably and bring home American troops in an orderly way so as to avoid a Vietnamese bloodbath. He turned McGovern’s pledge for peace into a prescription for disaster, charging that McGovern was a defeatist and unpatriotic and would forfeit for naught the tens of thousands of lives already lost, and create unmitigated havoc in Southeast Asia.

By election day, Nixon, always the master at figuring out where the votes were, had deprived McGovern the votes of Democratic cold warriors, the working class, and most independents, and had started the trek of Reagan Democrats toward the Republican Party.

McGovern’s domestic agenda was not much different from Obama’s: he proposed giving a $1,000 federal grant to every man, woman, and child, which he coupled with a call to boost tax rates on corporations and the wealthy. He wanted to cut the defense budget by 37% and spend it instead on social programs. Nixon, who had imposed wage and price controls just months earlier, accused McGovern of being a socialist, further pushing him outside of the political mainstream.

Senator McCain should follow Nixon’s lead. War hero McCain can turn Mr. Obama’s proposal to end the Iraq war into appeasement, charging that his policies would do to Iraq what McGovern’s would have done to Vietnam. Doing so would assure Mr. McCain that the issue in the campaign would not be the wisdom of the war itself or how we got into it, but how we end it with honor and without subjecting Iraq and the rest of the Middle East to more bloodshed.

The 1972 Democrat convention was one of the most dysfunctional meetings in American political history. Following the tumultuous 1968 convention in Chicago, new rules were developed, largely by McGovern himself, giving more power to left-leaning political activists and minorities, and less to rank-and-file Democratic constituencies, elected officials, and labor unions. Traditional power brokers were disenfranchised and replaced by neophyte left-wingers, the most notable being the mayor of Chicago and Democratic stalwart, Richard Daley, who was booted out of the 1972 convention and replaced by Jesse Jackson. Out of the fracas came a badly fractured Democratic Party unable to rally behind its nominee.

McGovern’s subsequent defeat by Nixon produced an identity crisis for liberals — should they stick to their principles and convictions or should they be willing to compromise in order to win elections on the other? It is an identity crisis, according to some, from which Democrats have never recovered.

The origins of that crisis go back long before 1972. As I point out in my book “Upstream: The Ascendance of American Conservatism,” liberalism started coming unglued soon after World War II ended, as conservative ideas became prominent and began challenging liberalism at every turn. After liberal icon Hubert Humphrey was defeated by Richard Nixon in 1968, it was inevitable that liberalism would soon be handed a crippling defeat, which indeed it was in 1972.

Exit polls taken after Super Tuesday indicate that between 20% and 30% of Democrats will be dissatisfied if their candidate is not the nominee. Should the battle between Mrs. Clinton and Mr. Obama continue on into the summer, those numbers will only get worse. If no nominee is named until the convention, “disgusted” rather than “dissatisfied” could be closer to the mark, making it difficult for either Mrs. Clinton or Mr. Obama to rally the party.

In either case, but especially if the nominee is Barack Obama, John McCain would be well advised to study the 1972 Nixon campaign, to show Mr. Obama to be the liberal that he is, and to reserve moderate Democrats and independents for himself.

Mr. Regnery, publisher of the American Spectator, is the author of “Upstream: The Ascendance of American Conservatism.”