Beto O'Rourke is youthful and has little political record to run on. Joe Biden is older and has a decades-long history, featuring both successes and mistakes. O'Rourke brings a viral energy to his campaigns, while Biden, still figuring out how to harness social media, specializes in the traditional stemwinder.

Both men are sending increasingly strong signals they will soon enter the presidential race - additions that would instantly reshape the Democratic primary debate. While left-leaning candidates have drawn the bulk of the attention so far, the popular former vice president and the charismatic former Texas congressman share one key characteristic - a more centrist brand of politics that could appeal to an untapped portion of the party electorate.

In targeting many of the same voters, O'Rourke and Biden could be on something of a collision course. They would present the party with the sharpest test yet of whether it wants an exciting but untested face or a more traditional, experienced standard-bearer to take on President Donald Trump. And both would be trying to lay claim to the Obama legacy, with Biden touting his eight-year partnership in the White House and O'Rourke attempting to tap into the cultural and generational zeitgeist as Barack Obama did more than a decade ago.

Biden, speaking Tuesday to the International Association of Fire Fighters, was greeted with chants of "Run, Joe, run!" He told the union members he appreciated their energy and urged them to "save it a little longer - I may need it in a few weeks." He added jokingly, "Be careful what you wish for."

O'Rourke plans a trip to Iowa later this week, suggesting he is laying plans for that early caucus state in the aftermath of an attention-grabbing appearance at the South by Southwest festival in Texas.

The potential candidacies hardly mean the other Democrats - many of them with strong followings and campaign skills - will fade. But Biden's long history in the party and O'Rourke's recent challenge to Sen. Ted Cruz, R-Texas, which electrified many Democrats, mean they are likely to be major forces.

And their messages would differ notably from candidates such as Sens. Bernie Sanders, I-Vt., Elizabeth Warren, D-Mass., and Kamala Harris, D-Calif., who have presented forceful liberal platforms.

They are three decades apart, but Biden, 76, and O'Rourke, 46, are Irish American white men who would be entering a field that is the most diverse in history in a party now energized by minorities and women.

The argument put forth by supporters of each of them is that they would appeal to white working-class voters who were attracted to Trump. Even before their candidacies are official, Biden and O'Rourke have been honing messages that stress optimism and unity.

"We seem to be at each other's throats. Extremism is on the rise in this country," Biden said in Tuesday's speech. "Mean pettiness has overtaken our politics. Sometimes it seems like we can't govern ourselves or even talk to one another. . . . Folks, it's not who we are."

O'Rourke has distanced himself from Democrats who have offered sweeping liberal policies, and he bristled when asked whether he considered himself progressive, saying he doesn't know and dislikes labels.

O'Rourke could announce a presidential bid any day and has been running hundreds of variations of an ad on Facebook, encouraging users to sign up to learn about his decision on whether to run.

"People in communities across the country have been reaching out and asking me if I'm planning on running in 2020," one ad says. "Sign up today to be first to know what's next. I'd like for you to be a part of it."

The challenge they present to each other is clear. Biden's experience and fluency with policy could make O'Rourke seem insubstantial and shallow. O'Rourke's youthful charisma could highlight Biden's age, making him seem like a disconnected figure past his prime.

In his appearances, Biden's references skew older, and he likes to tell war stories about his long tenure in the Senate.

On Tuesday, Biden started his speech by quoting Adlai Stevenson - "He said, 'Flattery is fine as long as you don't inhale' " - while jokingly stressing that he did not actually know the longtime pol, who died nearly 54 years ago. Biden also recalled battling alongside the firefighters in a dispute from 1986, when O'Rourke was in middle school.

O'Rourke likens his campaign to the punk bands he used to tour with, and he has been riding around El Paso, Texas, on a hip Surly bike. When he visited the South by Southwest festival, some on Twitter captured him on a street corner around midnight. The next morning he was in a T-shirt, posing for photos at an airport terminal.

While O'Rourke inspired legions of supporters during his 2018 Senate race, he lacks a signature accomplishment during his six years in Congress. And as candidates such as Warren churn out policy proposals, he would have a lot of catching up to do in outlining his own plans.

Biden has a different and in some ways opposite problem. He has been in public life for longer than O'Rourke has been alive, and he was elected to the Senate six weeks after O'Rourke was born. The Democratic Party has shifted dramatically even in the two years since Biden left office, with a surge of confrontational liberal activism.

Biden's challenges were on display last week, in the wake of a Washington Post report that resurfaced comments he made in 1975. In those remarks, Biden argued against busing children for desegregation - in colorful, vehement terms.

Harris, the daughter of Indian and Jamaican immigrants, took a different view in talking recently to reporters in South Carolina.

"I will just speak on behalf of myself - and this is not a reaction to statements he made decades ago - but there is no question we need still to integrate the schools of our country," she said.

Biden leads in most national Democratic polls. In Iowa, a recent poll showed him narrowly ahead of Sanders. Almost as important, Biden was the second choice for more voters than anyone else, which could be a critical factor in the state's caucuses.

But while other candidates have begun building organizations, Biden has been slow to do so. His supporters say his stature would enable him to move quickly.

"The big question is whether he can catch up on staff and whether he's going to focus on Iowa or not," said Sean Bagniewski, the chairman of the Polk County Democrats.

Still, he added, the 2016 race showed that staff and fundraising matter only to a point.

"Bernie Sanders wasn't the most fundraised and most staffed candidate. Donald Trump wasn't the most fundraised and most staffed candidate," Bagniewski said. "Money and good staff are important, but having a charismatic superstar candidate or a national message and stature matter more."

Biden won't inherit Obama's political network - or even get his blessing. Obama has been telling the presidential candidates he does not plan to endorse in the primaries, and he wrote on Facebook last week that it would help the party to have a long primary that could improve the ultimate candidate.

That message differs from what Biden says Obama told him four years ago. "In January 2015 the president was convinced I could not beat Hillary, and he worried that a long primary fight would split the party and leave the Democratic nominee vulnerable in the general election," Biden wrote in his recent book.

During O'Rourke's 2018 Senate campaign, Obama several times offered to help, saying he would go to Texas for a rally or to record robo-calls. He even recorded a video, though O'Rourke's campaign ultimately did not use it.

O'Rourke said in an interview with Oprah Winfrey that after the campaign, Obama gave him advice on how to decide on a presidential bid.

"He said, 'Look, just be very clear that this is one of the most intense' - I don't know if he used this word, but I took this from it - 'one of most brutal things you can go through,' " O'Rourke recounted.

Notably, O'Rourke and Biden are arguably best known for campaigns they lost. O'Rourke fell short in his 2018 challenge to Cruz, though Democrats were excited that he came within a few points. And Biden has twice run for president, garnering little support either time.

New Hampshire House Speaker Steve Shurtleff, D, who worked on Biden's 2008 presidential campaign, noted that Biden dropped out after getting less than 1 percent of the vote in Iowa, never even making it to New Hampshire.

"We never found out how he'd do," Shurtleff said. "Maybe now we'll find out."