Just ask Tim Kaine.
In 2008, Barack Obama considered Kaine, then governor of Virginia, as a potential vice presidential pick. Now, having served four years as a U.S. senator, Kaine is perhaps in a stronger position politically than he has ever been.
And he’s one of Clinton’s earliest supporters.
In May, Kaine headlined the annual Jefferson Jackson Dinner, an important date on South Carolina Democrats' calendars, to give a full-throated endorsement of Clinton and his support for the pro-Clinton PAC Ready for Hillary.
“Our country will have no shortage of challenges to tackle in the future. ... And the very list of challenges on the table tells us who the next president should be,” Kaine said.
Clinton, perhaps more so than any other American politician, keeps exhaustive records of political friends and foes — and such an early show of support likely won’t hurt Kaine’s standing.
Loyalty would be one of a host of factors for Clinton to consider in a vice presidential pick should she seek — and win — the Democratic nomination. Others would include geography, age and experience.
Often, selecting a running mate is viewed as an opportunity to address some of a candidate's most pressing weaknesses. Republican John McCain, who in 2008 saw his numbers flagging among women, chose Gov. Sarah Palin to join his ticket in a bid to win back some of their support.
Were Clinton to win the nomination, her team would likely gravitate toward picking a younger male lawmaker. The tougher decision would be whether to pick someone who would satisfy the progressive wing of the Democratic Party, or to play it safe.
Kaine, 56, could well be the favorite, safe choice at this early stage. But a number of other Democrats are well-positioned and well-groomed for the slot should the political winds shift in their favor.
In this cycle, the Senate will likely be fertile source for potential running mates -- and Clinton, having served there, might feel more comfortable with a pick from the eastern side of the Capitol. In addition to Kaine, Sen. Sherrod Brown, of Ohio, an important battleground state, has already been the target of some early speculation: On MSNBC earlier this year, host Chris Matthews told Brown, 61, flat-out that he wants him to run as Clinton's No. 2.
"I'm really happy where I am but, thank you,” Brown said.
Matthews nevertheless wrapped the interview by calling Brown “Mr. Vice President.”
Sen. Michael Bennet, of Colorado, might not yet have a cable-news endorsement, but he has other factors working in his favor. At 49, Bennet would be a younger complement to Clinton, and he hails from another important battleground state. Bennet also chairs the Democratic Senatorial campaign Committee, where he works alongside Executive Director Guy Cecil, who is thought to be in the running for Clinton's campaign manager.
Clinton might also look to the nation's governor's mansions, particularly if she finds herself facing a Republican governor. Maryland Gov. Martin O'Malley is weighing whether to run for president himself; if he makes the race, it could be a launching pad to Clinton's ticket should she clinch the nomination. And next door, in Delaware, Gov. Jack Markell would be wrapping up his second term in 2016, with high approval ratings and a record of pushing party priorities, including a higher minimum wage, in the state and in LGBT precincts.
The biggest knock on these governors could be geography: Could Americans stomach an all-East Coast ticket?
It's difficult to imagine Clinton selecting a wild-card pick as her running mate, but Julian Castro, 39, the former San Antonio mayor soon to take over at HUD, would make a strong case as a prominent Hispanic politician and rising star in the Democratic party. His lack of government experience -- San Antonio has a weak-mayor system -- would count against him.
Clinton could also shatter expectations by selecting a woman, such as Sen. Kirsten Gillibrand, of New York, who is Clinton's successor in the Senate.
And if two white women from New York on the same ballot sounds preposterously homogeneous, a similar tack has worked for another presidential ticket: Bill Clinton and Al Gore in 1992.
"I have deep concerns about the ticket,” the Rev. Jesse Jackson told The New York Times. “It takes two wings to fly, and here you have two of the same wing.”
What will fly for this Clinton will be one of the intriguing questions of the next election cycle.