Almost a month after the passing of Supreme Court Justice Antonin Scalia, President Obama's shortlist of potential nominees to replace him appears to be taking shape. Looking at lists from the Washington Post, the New York Times, the Associated Press, and NPR's Nina Totenberg, six candidates keep appearing:

Merrick Garland, US Court of Appeals for the DC Circuit

Ketanji Brown Jackson, US District Court for DC

Jane Kelly, US Court of Appeals for the Eighth Circuit

Patricia Millett, US Court of Appeals for the DC Circuit

Sri Srinivasan, US Court of Appeals for the DC Circuit

Paul Watford, US Court of Appeals for the Ninth Circuit

Reuters has reported that the list is even smaller: just Garland, Srinivasan, and Watford. Other options previously floated — like Attorney General Loretta Lynch, US Sen. Amy Klobuchar, and Adalberto Jordan of the Court of Appeals for the Eleventh Circuit — have either taken themselves out of contention or confirmed they're not being considered.

Here's what you need to know about the final list — their strengths, their flaws, and why Obama would and would not want to pick them — in subjective order of how much buzz they're picking up. Be sure to also read Ian Millhiser at ThinkProgress, who has useful details on all six.

Sri Srinivasan — US Court of Appeals for the DC Circuit

Srinivasan is by far the most-mentioned Supreme Court contender in DC, owing largely to his unanimous 2013 confirmation to the DC court. That's extremely impressive: The DC court is by far the most prominent circuit court, owing to its jurisdiction over challenges to rules made by federal regulators like the Environmental Protection Agency. The fact that Srinivasan could get unanimously approved for that court is truly remarkable.

His background helps explain the across-the-board enthusiasm. He worked for the solicitor general's office for seven years: five under George W. Bush, two under Obama. He clerked for two Republican judges, Sandra Day O’Connor and the Fourth Circuit's J. Harvie Wilkinson III.

Most notably, he's represented a number of corporate clients in private practice, including former Enron executive Jeffrey Skilling in his Supreme Court appeal, a newspaper that fired reporters for union activities, and Exxon when it was trying to escape liability for murder and torture its security forces committed in Indonesia. If anything, many liberal activists might think Srinivasan too conservative to promote to the highest court.

Then again, he would make history as the first Asian-American or South Asian justice, and the first Hindu justice. Moreover, he has a few liberal decisions under his belt already. He reinstated a Department of Labor rule extending minimum wage and overtime protections to home care workers after a lower court judge had blocked it, and declined to halt Obama's climate change rules (the Supreme Court later halted them itself). However, the latter case might be a reason for Obama to not nominate him. While the DC Circuit has ruled on whether to halt the rules, it hasn't ruled on the legal challenge itself yet, and Srinivasan is on the panel set to consider that.

"Nominating Srinivasan presents some risk for the president because it could lead to a different judge being swapped in to hear this case," Millhiser explains. "Should Srinivasan be confirmed to the Supreme Court, he would also need to recuse from the case because he already ruled on the request to temporarily halt the Plan as a circuit judge."

That hasn't been enough to stop Srinivasan from being considered, though. Obama has reportedly asked the FBI to conduct a background check on him, and the AP reports (and Reuters confirms) he's one of the three favorites for the position, alongside Watford and Garland.

Paul Watford — US Court of Appeals for the Ninth Circuit

At first glance, Watford is an immensely appealing, qualified nominee. He's only 48, and would be only the third African American appointed to the court. He also went to UCLA Law School, which might provide a valuable form of diversity at a time in which eight of nine justices went to Yale or Harvard (and Ginsburg graduated from Columbia after transferring from … Harvard). He clerked for the widely liked (albeit idiosyncratic) conservative judge Alex Kozinski, as well as Ginsburg. Conservative law scholar Eugene Volokh was an enthusiastic supporter of Watford's appellate confirmation.

What's more, he is by all accounts a very good judge. "In his three years on the court, he has earned a reputation as a smart and careful jurist," Totenberg writes. "Indeed, two of his opinions — one a dissent and one a majority opinion — were ultimately vindicated by the Supreme Court last term."

Those two cases were Los Angeles v. Patel, in which Watford ruled (and the Supreme Court affirmed) that a city code provision letting police officers look at hotel guest records without a warrant violated the Fourth Amendment; and Reed v. Town of Gilbert, wherein the Circuit Court ruled in favor of town sign regulations that set different rules for the size of "political" and "ideological" signs as opposed to ones providing directions, while Watford dissented and the Supreme Court followed his reasoning in striking down the regulations on First Amendment grounds.

That being said, a Watford confirmation is probably a long shot due to Grassley's strong opposition to his appellate appointment. Grassley objected to Watford's work on cases challenging Arizona's infamous 2010 immigration law and on an amicus brief critical of Kentucky's lethal injection protocol. He was only confirmed 61 to 34, with a Democratic Senate, suggesting his confirmation for a higher court under a Republican Senate is highly unlikely.

Still, the AP reports and Reuters confirms that Watford is one of Obama's three favorites for the vacancy, along with Srinivasan and Garland.

Merrick Garland — US Court of Appeals for the DC Circuit

If the Senate were Democratic and a liberal justice were up for replacement, it's unlikely Garland would be in serious contention. He's a white man at a time when the Obama administration is aggressively trying to make the federal bench more diverse, he's 63 at a time when presidents are trying to pick justices in their 50s to maximize their tenures, and he's very pro–law enforcement at a time when the administration is looking to rein in the police. He even joined in a ruling that effectively barred Guantanamo detainees from seeking relief in US courts (a decision the Supreme Court then reversed), though his defenders insist he was bound by precedent there.

And he has no shortage of conservative fans. In 2010, when he was being considered for the seat for which Elena Kagan was ultimately nominated, the conservative judicial appointments activist Carrie Severino offered measured praise for Garland: "Of those the president could nominate, we could do a lot worse than Merrick Garland. He's the best scenario we could hope for to bring the tension and the politics in the city down a notch for the summer." Then-Senate Judiciary Committee ranking member Orrin Hatch said there was "no question" Garland could be confirmed.

But all of those are potentially factors that make him more palatable to a Republican Senate. His age means his tenure is limited; it's not like approving a John Paul Stevens who'll serve for 35 years. His career as a prosecutor and his pro-police rulings reduce the odds that a liberal court will overturn existing precedent on criminal justice issues, which is significant given that Ruth Bader Ginsburg and Stephen Breyer have both suggested they want to strike down the death penalty entirely.

And you certainly can't knock him on experience. He's been on the DC Circuit for 19 years and has served as its chief judge for three. All that helps explain why Obama has asked the FBI to conduct a background check on him, and why the AP and Reuters both put him, Srinivasan, and Watford as the three favorites.

Jane Kelly — US Court of Appeals for the Eighth Circuit

Any nominee is going to have to go through the Judiciary Committee, chaired by Sen. Chuck Grassley (R-IA). And no nominee is going to make Grassley happier than Jane Kelly, a career public defender from Iowa whose nomination for the federal bench Grassley championed, leading to a unanimous confirmation in 2013. She was also, coincidentally, a Harvard Law School classmate of Barack Obama's, graduating with him in 1991.

Her record as a defense attorney might spark some objections from law-and-order-oriented conservatives in the Senate. "Kelly has joked that she cannot remember a time when she did not know how to make methamphetamine and that she has spent enough time in prison to serve out sentences for several misdemeanors," according to a since-deleted passage in a New York Times piece on her potential Supreme Court nomination.

But it's hard to argue she lacks empathy for victims of crime. In 2004, while jogging, she was tackled and beaten by a male stranger, requiring months of recovery before returning to her practice.

The New York Times has reported that Kelly is being vetted by the Obama administration and background-checked by the FBI. When asked about her potential appointment, Grassley was muted, suggesting he could support Kelly but perhaps not if Obama rather than Hillary Clinton appoints her:

Mr. Grassley, in the interview on Wednesday, said he hoped Judge Kelly would be on a short list of potential Supreme Court nominees for the next Democratic president. "In this particular instance," Mr. Grassley said about the election-year vacancy, "it has got to be the process, and the person doesn’t matter, see." He also broke with the Senate majority leader, Mitch McConnell of Kentucky, who has flatly ruled out meeting with the president’s nominee. Mr. Grassley said that he had not yet decided whether he would do so, and that Judge Kelly, as an Iowan, would be welcome in his office any time. "You know, one of the questions I will ask them," he said of the eventual nominee, will be "what they feel about being used as a political pawn." "If I talk to them in my office, I’d do that," he said.

Totenberg classifies Kelly as a "longer shot," owing to both her relatively short time on the bench and the potential for Republicans to dredge up old public defender cases. The latter is already happening, as Severino, the conservative legal activist, has already attacked Kelly for having represented a client charged with possessing child pornography.

Ketanji Brown Jackson — US District Court for DC

Brown Jackson was a fairly surprising name to see pop up on the White House shortlist. For one thing, she's a district court judge, not an appellate judge. Nine of the past 10 justices nominated were federal circuit court judges first (Kagan is the exception), and the last person appointed directly to the Supreme Court from a district court was Edward Terry Sanford, whom Warren Harding put on the bench in 1923.

Furthermore, Brown Jackson has only been on the DC court for three years, and received a "qualified" but not "well-qualified" rating from the American Bar Association upon her nomination — "enough to get her confirmed but not enough to suggest that she was at that time a legal star," per Totenberg.

But all the same, she has a lot going for her. While her judicial experience is light, she is a past Supreme Court clerk (for current Justice Stephen Breyer) and spent years on the US Sentencing Commission, providing an inside perspective into criminal justice policy that's rare on the Court. She's 45, meaning she could potentially serve for many decades, maybe even surpassing William O. Douglas's record of 36.5 years.

And she would be the first black woman ever appointed to the Court, something for which many civil rights activists have been pushing. One who met recently with the administration told the Washington Post that it would be "heartbreaking" if Obama got three Supreme Court picks and "none of them were African American." Veteran Supreme Court litigator and SCOTUSBlog founder Tom Goldstein has predicted that Brown Jackson will be the nominee because Republican obstruction of her could mobilize black turnout in November: "It is easy to see a political dynamic in which candidate Hillary Clinton talks eagerly and often about Judge Brown Jackson in the run-up to the 2016 election, to great effect."

Brown Jackson also has a family connection that might help her out: She's related by marriage to House Speaker Paul Ryan. Her husband, Patrick Jackson, is the twin brother of William Jackson, who's married to Ryan's wife's sister. Ryan even testified on her behalf when she was nominated for the district court, saying: "Our politics may differ, but my praise for Ketanji’s intellect, for her character, for her integrity is unequivocal. She’s an amazing person, and I favorably recommend her consideration."

Millhiser notes that Brown Jackson has two cases in her past that could cause trouble. She ruled against Gawker in a suit attempting to "force former Hillary Clinton aide Philippe Reines to explain why he had work-related emails in a private account"; given that Clinton email issues are likely to surface in the general election, this is a very plausible Republican attack line. She also ruled against a company challenging a Defense Department program for small businesses that considers whether the businesses are minority-owned in its selection criteria; this can be construed as a ruling for affirmative action.

According to Totenberg, Brown Jackson is considered a "longer shot" for the nomination, and the AP characterizes her as a "less likely" pick than Srinivasan, Watford, or Garland. But she does bring some strengths to the table that no other finalist does.

Patricia Millett — US Court of Appeals for the DC Circuit

Millettt is, along with Nina Pillard and Robert Wilkins, one of the nuclear option trio, the three judges nominated for the DC Circuit in 2013 who were initially blocked by Senate Republicans and then provided the pretext for then–Majority Leader Harry Reid to blow up the filibuster for executive branch and non–Supreme Court judicial nominations.

Before that she had a long career as an appellate litigator, in particular at the Supreme Court. She's argued 32 cases before the Court, 25 during her 11 years at the Solicitor General's office and seven in private practice at the firm Akin Gump. She briefly held the record for most arguments before the court by a woman litigator; her friend Lisa Blatt has since surpassed her.

On the merits, Millett is a politically compelling nominee. She's married to a longtime Navy veteran and was supported in her appellate court nomination by military spouse organizations. She has a reputation as being favorable to corporate interests due to both her representation of a variety of corporate clients at Akin Gump and her defense of the Roberts Court's business record. She is outspoken about her Christian faith.

But her nomination would bring the filibuster reform battle, which has left many GOP senators still bitter, back to the fore. Ultimately she wound up being confirmed with only 56 votes and only two Republicans voting in favor. "If Millett is the nominee," ThinkProgress's Millhiser notes, "it is likely that many senators will take their frustrations with this rules change out on the judge."