With a month to go until the Democratic National Convention, Hillary Clinton’s campaign is weighing a slate of potential running mates, conducting interviews with candidates and in many cases requesting personal financial and medical information.

Virginia Sen. Tim Kaine, a well-liked swing state lawmaker who was a finalist on Barack Obama’s vice presidential shortlist in 2008, has emerged as an early frontrunner for the job. Also under consideration are Massachusetts Sen. Elizabeth Warren, Ohio Sen. Sherrod Brown and Housing Secretary Julian Castro. But officials with knowledge of the process say that the campaign is sifting through an even longer list, pondering some outside-the-box prospects as well as more widely-circulated names like New Jersey Sen. Cory Booker and Labor Secretary Tom Perez.

Geography and demographics are important – particularly as Democrats eye the possibility of taking control of the Senate, where they covet having a majority in place for the next administration’s crucial first 100 days. But Democrats familiar with the process say that the most important criteria for Clinton is finding a compatible partner whom she likes and trusts – and who is ready to step into the top job.

Here is a list of each potential candidate and what we know:

TOP TIER

Tim Kaine

A personable former governor, Kaine is viewed as a solid and competent pick with the experience and temperament to pass the commander-in-chief test – a significant factor in Clinton's decision. He speaks fluent Spanish, governed a purple state and has foreign policy chops from his years on the Senate Foreign Relations Committee. A past chairman of the Democratic National Committee, he also knows the ins and outs of party fundraising and political strategy. A former missionary who attends a black church and was one of Clinton’s earliest endorsers, Kaine is also one of only about 20 people in American history to have served as mayor, governor, and senator. Another plus: Virginia has a Democratic governor who would appoint his replacement, so his departure from the Senate wouldn’t be a net loss for the party. On the other hand, some in the Clinton camp worry he doesn’t have the “zing” to excite voters.

Elizabeth Warren

If Clinton determines that she needs to mollify disgruntled Bernie Sanders fans and motivate younger and more progressive voters, Warren would be an obvious choice. Sources indicate that she’s definitely on the list, though the arguments against her are also compelling. 1) A all-female ticket could introduce some risk; 2) she’d almost certainly be replaced -- in the short term -- by an appointee of a Republican governor; and 3) she doesn’t have a long-standing personal relationship with Clinton, although she’s clearly filling the role of a top surrogate now. Another factor is that Clinton’s team believes that Warren is already an effective attacker of Trump from her current perch. With Warren poised to have even more influence in the next Senate, she may ultimately prefer to stay there.

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Sherrod Brown

Brown’s biggest drawback is that his departure from the Senate would put a swing state Senate seat back into Republican hands. Particularly with Florida looking much more competitive now that Marco Rubio is running for re-election, Clinton would have to weigh whether the advantages of putting someone on the ticket from swing state Ohio outweighs the risks of dropping a Senate seat there and jeopardizing a Democratic takeover. That said, Clintonworld has been impressed with Brown as a surrogate so far, and he’s a progressive who could fire up base voters.

IN THE HUNT

Cory Booker

Booker is young, dynamic and African-American. But he’s less experienced than many of the other picks, with less than three years under his belt in the Senate. He has a strong personal story to tell; he’s a former Stanford football player-turned Rhodes Scholar-turned Newark mayor-turned U.S. senator. But his strong ties to Wall Street could alienate progressives already wary of Clinton’s relationship with the banks. And, like Brown and Warren, Democrats would lose a Senate seat to a Republican appointment if Booker gives it up. Still, he’s impressed Clinton and her aides with the passion and energy he brought to campaigning for her in Iowa.

Julian Castro

The Housing and Urban Development Secretary is telegenic and Hispanic, and he’s had a good rapport with Clinton when they’ve been together on the stump. But his lack of experience – which detractors say shone clearly through during his handling of a fight with the left on mortgage policy – could lead critics to label him a lightweight, and it’s not clear that Clinton’s team thinks he’s ready for the job.

Tom Perez

Clinton is said to like his “fire” on the trail, and he’s a Latino progressive who could mobilize some key constituencies. But, as with Castro and Booker, experience could be a problem for a man who was still only a local lawmaker only a few years ago. That said, his tenure heading the Labor Department, and earlier at as the Assistant Attorney General for Civil Rights, has received glowing reviews from many Democrats and labor union officials.

Xavier Becerra

Becerra has clearly demonstrated his willingness to be Clinton’s running mate. He’s the highest-ranking Latino in the House of Representatives and has been a reliable Clinton ally. But he’s got little in the way of foreign policy chops, and few outside Washington know who he is.

ON THE BUBBLE

Al Franken

If Clinton’s team likes the snarky attack-dog style of Elizabeth Warren but thinks the Massachusetts senator is too risky, Franken could be an outside-the-box alternative. The Minnesota progressive has policy chops that would please the Sanders crowd, and his “Saturday Night Live” background could come in handy countering Donald Trump’s jabs with comedic routines of his own. And, like Kaine, his departure wouldn’t forfeit a Senate seat. But if Clinton’s painting herself as the serious choice in the race, is America ready for a former “SNL” comedian to be a heartbeat away from the presidency?

Shaun Donovan

The former HUD secretary and current budget director has good credentials on the economy – an area of policy ground where Trump has tended to lead Clinton in head-to-head polls. But it’s a tough sell to say that a largely unknown housing and budget expert – who hasn’t ever been elected to any political office – should be the second in command of the United States.

Mike Mullen

The former chairman of the Joint Chiefs of Staff is a dark horse pick who could bring military and foreign policy credentials to the table. He and Clinton worked together during her tenure as Secretary of State, and he was once vetted by Michael Bloomberg’s team, so he’s not a total novice to the process. But it’s not clear that Clinton wants to double down on foreign policy rather than finding a more well-rounded or well-known pick.