AUSTIN -- A few years ago, Doris Kearns Goodwin, the biographer of Lyndon Johnson and the Kennedys, told a group of reporters at the American Press Institute a story that should be drilled into the consciousness of every journalist and politician in this country.

When she was working with Johnson on his memoirs, Goodwin said, she summoned the courage one day to ask him why he had so often told people that his grandfather had died at the Alamo, when he knew it was not true.

Johnson, she said, confessed to the fraud, but had a ready explanation. "My grandfather wasn't killed at the Alamo," she recalled him saying. "He was killed at the Battle of San Jacinto, which Texans know was more important in achieving independence {from Mexico}, but other Americans usually haven't heard of. So I moved him to the Alamo."

"That seemed reasonable," Goodwin said, "until I did some further checking and found he hadn't been killed in the Battle of San Jacinto either. He died in bed."

It was a funny story, but the historian had a serious point to make: "When you find discrepancies in the stories of politicians, pay attention." Someone who refuses to deal honestly with his private life may well distort the reality he confronts in public office, she said. In a chilling example, she recalled how Johnson as president had turned what she called "an ambivalent incident in the Gulf of Tonkin" -- an encounter between a U.S. ship and a North Vietnamese vessel, which Johnson called an unprovoked act of aggression -- into the pretext for an escalation of American forces in Vietnam that "cost thousands of lives and caused bitter divisions in our country."

Johnson is buried in his beloved Hill Country, west of here, but sadly, the lesson Goodwin drew from his life is still being painfully learned. The main political news in Texas last week was the revelation that Lena Guerrero, one of the state's most personable and promising young politicians, had inflated her academic record outrageously in official biographies since entering public life.

Guerrero, who served three terms in the Texas house before her appointment last year as chairman of the Texas Railroad Commission, the powerful body that regulates the state's energy industry, has repeatedly identified herself in official and campaign literature as a Phi Beta Kappa graduate of the University of Texas.

Last weekend, as the Dallas Morning News prepared to reveal the truth, Guerrero called a news conference to say she had left school four credits short of graduation and had not been chosen for the academic honorary society. Four days later, under continuing pressure, she made public a transcript that showed she was 19 hours -- almost a full semester -- shy of meeting degree requirements, because she had flunked six courses. Her explanations for the discrepancies were notably unpersuasive.

Guerrero is a personal protege of Texas Gov. Ann Richards (D), and Richards had gone to extraordinary lengths to help ensure that the woman she appointed to the Railroad Commission would be confirmed by the voters this November, thus becoming the first Hispanic woman elected to statewide office in Texas.

With no Senate or gubernatorial contest on the ballot, Guerrero's race against Republican Barry Williamson was counted on by Democrats to help spur a big turnout in Hispanic areas for the presidential ticket. Her embarrassment is a blow to Bill Clinton's strategy in this vital state.

Both the Johnson and Guerrero cases suggest why it is not just journalistic torture -- as some of my letter-writers seem to think -- to keep probing the truthfulness of President Bush's account of his role in the Iran-contra affair and Clinton's history of his dealings with the Vietnam War draft.

Some correspondents suggest that the recent disclosure of a 1987 memo from then-Secretary of State George Shultz, in which Shultz said that then-Secretary of Defense Caspar Weinberger questioned Bush's disclaimer of any involvement in the arms sale debate, is far more serious than Clinton's varying accounts of how he stayed out of uniform during the Vietnam War. The actions of a 63-year-old vice president and member of the National Security Council are much more important than those of a 23-year-old youth with no public responsibilities at all, they say.

I have to disagree. This is not an either-or choice. It is important to test Bush's veracity, but it is also important to test the truthfulness with which the 45-year-old Clinton of 1992, a man who might be president, recounts what was clearly a central and personally troubling episode in his own life.

It would be wonderful if politicians instinctively told the truth about their life histories. But when they don't, journalists have no choice but to press the issue. That's not nitpicking. It's necessary work.