With the Democratic Convention in Philadelphia next week, Hillary Clinton's sidekick is expected to be revealed in the coming days. Who is in the running?
Donald Trump went for a safe option in Indiana Governor Mike Pence.
Unlike the Republicans, Hillary Clinton's list has been fairly stable.
That doesn't mean that she won't pick a dark horse, but let's be honest - excessive risk-taking isn't her style (at least, outside of the realm of email servers).
Who is he? The junior senator from the state of Virginia. Prior to being elected to Congress in 2012, Mr Kaine served as governor of Virginia and mayor of Richmond, the state's capital. He's fluent in Spanish and served as a Catholic missionary in Honduras during law school. As a senator he's focused on foreign policy and military issues.
Why he's the choice: He has an impressive resume. Virginia is a key swing state for Democrats. And he could appeal to Catholic voters, which are an important constituency in another nearby electoral battleground, Pennsylvania. Virginia has a Democratic governor (who would have the power to name an interim replacement if Mr Kaine has to vacate), so he wouldn't be replaced by a Republican in the Senate - possibly leaving the chamber's control in conservative hands.
Why he isn't: He's not the most animated politician, so he wouldn't help Mrs Clinton there. The biggest obstacle, however, may be his views on abortion. He's personally opposed to the procedure, and although he supports the Roe v Wade decision legalising it nationally, he's in favour of parental-notification laws and a ban on late-term abortions when the mother's health isn't at risk. Although he's recently moved to the left on the subject, those prior positions will make him a hard pill to swallow for pro-choice groups.
Zurcher's Odds: 3-1
Who is he? The secretary of housing and urban development. Mr Castro has been on the fast-track to prominence in the Democratic Party ever since he was elected mayor of San Antonio in 2009. He was given a prime speaking spot in the 2012 Democratic National Convention, and his appointment to a Cabinet position in the Obama administration in July 2014 was viewed as a move to groom him as a possible vice-presidential pick.
Why he's the choice: He's young, and he's Hispanic. Despite the support that this growing bloc of voters has given Democrats in past elections, the party's stable of prominent Latinos is remarkably thin. Although Mrs Clinton will likely have a decided advantage over Donald Trump with the demographic once again, someone like Mr Castro on the ticket could help cement its support for a generation to come.
Why he isn't: He's young, and he's inexperienced. He's not exactly the most compelling public speaker, and the position of mayor of San Antonio is heavy on ceremony and light on actual power. Mr Kaine is more fluent in Spanish than he is. He hasn't done much of note as housing secretary and has drawn criticism for a department policy of selling delinquent home mortgages to Wall Street investors.
Who is she? The senior senator from Massachusetts and a star of the populist left. In 2010 she helped the Obama administration set up the newly created Consumer Finance Protection Bureau. Ms Warren left her Harvard University professorship in 2012 to run against incumbent Senator Scott Brown, who himself had won an upset special election in 2010 to replace the powerful Democratic Senator Edward Kennedy when he died. She's since made a name for herself in the Senate as a passionate critic of Wall Street and a formidable presence during finance-related congressional hearings.
Why she's the choice: If Mrs Clinton feels she needs to shore up her left flank following her gruelling battle against Vermont Senator Bernie Sanders in the Democratic presidential primary, Ms Warren would be an obvious pick. She's also shown a singular ability to get under Mr Trump's skin with her blistering attacks on his ethics and business acumen.
Why she isn't: Ms Warren is practically the same age as Mrs Clinton, so she's not suited to be a party stand-bearer in future presidential elections. Mrs Clinton may not want to move further to the left heading into a general election battle for moderate voters. There's also the concern that having two women on the ticket could be a gender bridge too far, although the kind of voters who aren't interested in an all-woman ticket probably aren't keen on a one-woman ticket either. And Massachusetts has a Republican governor, so a Warren victory could at least temporarily help keep the Senate out of Democratic control.
Home: New Jersey
Who is he? The junior senator from New Jersey. Prior to his election to Congress in 2013, he served as a popular, reform-minded mayor of Newark, a large New Jersey city near New York. In 2012 he made national headlines for saving a woman trapped in her burning house. He attended Oxford as a Rhodes Scholar and has been mentioned as a possible Democratic Supreme Court nominee.
Why he's the choice: He's young and charismatic. Of all the top-tier vice-presidential possibilities, he has the best stage presence and energy. He's also black - and could help Mrs Clinton maintain the high levels of minority support that Barack Obama enjoyed in his two presidential runs. In 2012 Mr Obama won 93% of the black vote, compared to 88% for Democrat John Kerry in 2004.
Why he isn't: Mr Booker has close ties to Wall Street financers, having received $1.88m from the securities and investment industry in 2014 - more than any other US senator. In a 2012 television interview he called Mr Obama's criticisms of private equity firms "nauseating". Those are serious strikes against him in today's populist-dominated Democratic Party. Like Ms Warren, a Republican governor (Chris Christie) would get to pick his replacement if he wins.
The rest of the field
Sherrod Brown of Ohio is another senator who, like Ms Warren, could appeal to the progressive Democratic vote. So could comedian-turned-Senator Al Franken of Minnesota. Labour Secretary Thomas Perez and California Congressman Xavier Becerra are the two other Hispanics getting some play in the press.