Joe Biden beamed from ear to ear in a Washington hotel ballroom Tuesday as firefighters chanted "Run, Joe, Run!"

"Save it a little longer," the former vice president told the union members. "I might need it in a few weeks."

He may need more than that.

Biden's not-yet-announced campaign already has hit bumps on the road to the White House: unsought reminders of the twists and turns of his nearly half a century in politics.

If Biden runs, he will argue that he has the experience necessary to be president. But it's also his biggest handicap.

Biden has been a U.S. senator, vice president or former vice president for 46 years - longer than many voters have been alive.

He thus carries a weight no other candidate must bear: the burden of history. His toughest opponent might be his own past.

Decades ago, Biden often took positions that were in his party's mainstream at the time, but have since been overtaken by changing public attitudes.

In the 1970s, he opposed busing to desegregate schools. In the 1980s, he pushed tough sentences for drug crimes. Later he supported free trade agreements and voted in favor of the 2003 invasion of Iraq.

Last week, the Washington Post published a front-page story that cited a Delaware newspaper interview from 1975, when Biden was a first-term U.S. senator.

"I don't feel responsible for the sins of my father and grandfather. I feel responsible for what the situation is today, for the sins of my own generation. And I'll be damned if I feel responsible to pay for what happened 300 years ago," Biden said in a long explanation of his opposition to busing.

Biden's comments escaped national notice at the time. But his opposition to busing was no secret; the Post reported on it at least three times since 1987.

But they were probably new to many readers, especially anyone younger than 40 - and they sounded discordant amid current debates on racial justice and reparations.

Biden hasn't changed his views that busing was a bad way to achieve desegregation, a policy that has fallen into disuse in most of the country. He has a strong record on civil rights in virtually every other respect, broad support among African American voters - and served as vice president under the country's first black president.

"When it comes to Biden's standing on civil rights, I'd defer to Barack Obama," Richard A. Harpootlian, a South Carolina state senator who's long been a Biden supporter, told me.

On other issues, Biden has changed with the times. He once opposed gay marriage and voted for the 1996 Defense of Marriage Act. But as vice president, he publicly nudged Obama toward supporting same-sex unions.

Sometimes Biden has gone back and forth. He voted for the 1993 North American Free Trade Agreement, then switched sides and declared himself "a fair trader." As vice president, he flipped back to support Obama's proposed Trans-Pacific Partnership.

"You can't judge people by what they did 50 years ago," Harpootlian argued. "To measure what they said then by today's standards is just wrong."

But all that evolution poses a problem for a presidential campaign, even if the candidate's explanations are convincing.

If Biden steps into a debate with a dozen other Democrats, he'll have a big target on his back.

Bernie Sanders can savage him for accepting campaign contributions from bank executives. Elizabeth Warren can denounce his support for the 2005 law that made it more difficult for people to declare bankruptcy. Kamala Harris can question his vote to endorse George W. Bush's invasion of Iraq in 2003.

An ancient rule in politics: If you're explaining, you're losing.

Every controversy will remind voters that Biden, who is 76, has been around a long time. He's not the fresh face most Democrats have told pollsters they want to see in the race. He can't claim to be an outsider.

Even Biden's appeals to civility sometimes make him sound like a throwback.

A few weeks ago, he praised Vice President Mike Pence as "a decent guy." After gay rights activists complained, Biden apologized and withdrew the compliment - but he doesn't sound happy about it.

"Mean pettiness has overtaken our politics," he told the firefighters Tuesday. "I get criticized for saying anything nice about a Republican. Folks, that's not who we are."

It's a noble sentiment - but it's another reminder that Biden represents an older tradition.

Biden still enjoys enormous strengths as the race begins.

He leads every poll of potential Democratic candidates. While most of his rivals are leaning left, he has a strong base in the party's moderate wing, which includes most Democratic voters.

His favorability rating in the party is stratospheric - 76 percent in a Monmouth University poll this month, higher than any other candidate.

But favorability can melt away in the heat of a campaign, as Hillary Clinton found in 2016. Biden could see his past overshadow his efforts to set a path for the future.

If he jumps in the race, Biden will face a cruel irony: All his hard-won experience no longer counts as a virtue; it may be his downfall.

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