The U.S. presidential election has been jolted with just over 100 days remaining by Joe Biden's decision to become the first president in 56 years not to seek a second term, but there will still be greater-than-usual focus on the vice-presidential nominees.

The expected contest between Biden, 81, and Republican nominee Donald Trump, 78, put the spotlight on those who would be the proverbial "heartbeat away" from the Oval Office, as it featured the two oldest-ever campaigners for president.

Additional scrutiny was placed on the Republican choice. Vice-presidential pick J.D. Vance has said he disagreed with former vice-president Mike Pence's decision to defy Trump, when Pence certified the 2020 election win for Biden.

It's now the turn of the Democratic pick to be the subject of intense speculation.

In just over one day after Biden dropped out of the race, Democrats largely coalesced in support for current Vice-President Kamala Harris. Harris has already gained the backing of enough delegates needed to win the nomination for president in first-ballot voting of party members.

Someone with an extensive electoral and campaigning background could be valued to run alongside Harris given her record in national campaigns, which consists of one victorious Senate race representing California, a disastrous presidential campaign that ended before the first state primary and a 2020 campaign alongside Biden in which in-person events were curtailed by the COVID-19 pandemic.

Harris would be the first Black woman, the first South Asian woman and just the second woman overall to top a presidential ticket, meaning demographics could be important. Early polling has suggested that Harris shares the steep disadvantages faced by Biden when he ran against Trump concerning the voting preferences of white men with less than a college education.

In what could be nasty campaign — there have already been signs of sexism and racism emerging online — a candidate seen as a steady and moderate voice could be key. Trump's senior campaign advisers have already branded Harris in a memo as "dangerously liberal."

Interestingly, many of the names that have been put forth in U.S. media reports as VP contenders lack experience in the U.S. Congress. Republican Sarah Palin in 2008 was the first nominee of the two major parties since the 1970s to be an exception.

Based on tracking originally compiled by the Pew Research Center in 2016, 29 vice-presidential nominees had at one time been a U.S. senator and 12 had served in the House of Representatives. The VP performs some ceremonial duties in Congress, and one with experience can be helpful in serving as a liaison between the chambers and the White House on legislation under consideration.

Biden was reportedly convinced to bow out after being given polling information from leading party figures like Nancy Pelosi that showed him with insurmountable deficits in every battleground state.

Should Harris be the nominee, no one choice for her running mate will erase all of the Democratic challenges. Her choice for VP will likely be in place ahead of the party's convention on Aug. 19-22 in Chicago. Here are some possible candidates:

Kentucky Gov. Andy Beshear

Beshear, 46, has won two gubernatorial contests in a state that opted for Trump in the 2020 U.S. presidential election by more than 25 percentage points. Beshear has touted his record of bringing jobs to Kentucky, supporting public education and expanding health-care access. As governor, he has vetoed legislation banning abortion, but it was overridden by the Republican-dominated legislature.

Transportation Secretary Pete Buttigieg

A ticket featuring Harris and the gay, married Buttigieg, 42, would energize many staunch Democrats but perhaps prove challenging in gaining the support of undecided older voters. Buttigieg attended Harvard University and served for years in the U.S. navy, including a deployment in Afghanistan. After impressing many observers with his communication skills and grasp of issues in a longshot 2020 presidential bid, he has served as Biden's transportation secretary. The former South Bend, Ind., mayor now has his permanent residence listed as Michigan, the home state of his husband and a critical state for Democrats in November.

Texas Rep. Joaquin Castro

While white politicians have been most often mentioned as possible Harris running mates, the Texas-born politician of Mexican descent could help Democrats with a demographic bloc that Trump has made some gains with, based on polling. Castro, 49, possesses a decade of U.S. congressional experience and a decade prior to that in his own state legislature. He has not been a muted presence, with frequent appearances on national political shows to discuss challenges at the southern border, two hours from San Antonio, where he was born and raised.

North Carolina Gov. Roy Cooper

Cooper, 67, has won praise for his strong focus on economic development, an area that hasn't been a consideration in most of Harris's political positions. He has won six statewide general elections over two decades in a state where Republicans routinely prevail in the legislature, and in a state that favoured Trump in both the 2016 and 2020 elections. Cooper persuaded enough Republican legislators last year to expand Medicaid under the Affordable Care Act, a victory. Other efforts have been thwarted by a General Assembly with veto-proof majorities that have eroded his formal powers.

Arizona Sen. Mark Kelly

Kelly, 60, who has been viewed as a moderate since joining the U.S. Senate in 2021, offers a compelling narrative. The former U.S. navy captain and NASA astronaut is a gun owner who believes in sensible gun control measures — given the near-fatal shooting of his wife, former Congress member Gabrielle Giffords, by a person with mental health issues. Kelly is also well versed in the country's immigration challenges, representing a southern border state, but he spent about the first half of his life in the eastern part of the U.S., hailing from New Jersey.

Illinois Gov. J.B. Pritzker

Illinois is not believed to be a perilous state for Democrats, but it neighbours Wisconsin, a battleground state. Pritzker, 59, could attract needed support for the party from the business community, as he is a billionaire whose family's holdings include the Hyatt hotel chain. Twice a winner in gubernatorial elections, Pritzker is a staunch supporter of gun control and women's reproductive rights, and he could attack Republicans on free-speech issues, having signed legislation prohibiting book bans in libraries.

Pennsylvania Gov. Josh Shapiro

Shapiro, 51, took office just last year but is already seen as a rising star. Trained as a lawyer, Shapiro served as the state's attorney general and in his state's legislature. He is a staunch proponent of abortion rights and has positioned himself as a moderate on energy issues in the nation's No. 2 natural gas state. Shapiro, who is Jewish, has aggressively confronted what he viewed as antisemitism stemming from pro-Palestinian demonstrations since the Israel-Hamas war erupted last year.