Speculation has already begun as to whom Hillary Clinton Hillary Diane Rodham ClintonJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Momentum growing among Republicans for Supreme Court vote before Election Day Warning signs flash for Lindsey Graham in South Carolina MORE might pick as a running mate if she becomes the Democratic Party’s presidential nominee in 2016.

Clinton was asked point-blank about the issue during a recent public forum in New York City, with a Massachusetts senator and President Obama’s nominee as the next Secretary of Housing and Urban Development (HUD) posited as contenders.

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"Were you to run for president in 2016," ABC News' Robin Roberts asked, "would you consider Elizabeth Warren Elizabeth WarrenBiden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon MORE or Julian Castro for your running mate?"

Clinton wouldn't concede she was running, but she did praise Warren and Castro.

"They're both extraordinary leaders and great political advocates for a lot of what needs to be done in our country, and I admire both of them greatly," she said.

There are almost two-and-a-half years to go before Election Day, but Warren and Castro are only two of several clear players who have already emerged as possible running mates for the former Secretary of State.

1. Maryland Gov. Martin O'Malley

Many Democrats believe O’Malley will run for president if Hillary decides not to do so. If she goes ahead, O’Malley is seen as a strong contender for the Number Two spot on the ticket.

“Given all of the press he’s generated over the last six months or so, he is obviously aiming to raise his profile,” said strategist Jim Manley, a former spokesman for Senate Majority Leader Harry Reid (D-Nev.).

O’Malley is described by supporters as a charismatic and strong executive. But he also offers some drawbacks.

One: Doubts over whether he is truly willing to play second fiddle. Another: he is from Maryland.

“What he doesn’t bring to the table is a swing state that might otherwise not be in play,” noted Rep. Gerry Connolly (D-Va.).

Others say that geography gets too much attention in the media.

Phil Singer, who worked for Hillary Clinton’s 2008 presidential campaign, said that executive or legislative accomplishment would be top factors in Clinton’s pick, if she were to run and win the Democratic nomination. Race, gender and region would take a secondary role, he suggested.

Singer noted that Vice President Biden comes from Delaware while his predecessor, Dick Cheney, hails from Wyoming. “Hardly big swing states,” Singer said.

2. Sen. Mark Warner Mark Robert WarnerIntelligence chief says Congress will get some in-person election security briefings Overnight Defense: Trump hosts Israel, UAE, Bahrain for historic signing l Air Force reveals it secretly built and flew new fighter jet l Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' House approves bill to secure internet-connected federal devices against cyber threats MORE (D-Va.) or Sen. Tim Kaine Timothy (Tim) Michael KaineBarrett seen as a front-runner for Trump Supreme Court pick Biden promises Democratic senators help in battleground states Second GOP senator to quarantine after exposure to coronavirus MORE (D-Va.)

Virginia’s two senators are both former governors of the crucial swing state.

“Mark is slightly to the right of Tim on some issues but they both have extraordinary executive-level achievements in a hard-to-govern state. And they are both terrific people,” Rep. Jim Moran, another Democrat in the Virginia delegation, said.

Moran said that Warner may very well want to be president in his own right, but only if Clinton steps aside.

“I don’t think he would ever run against Hillary but I think he would be more than happy to team up with her, as would Tim,” Moran said.

Strategists said that Warner has advantages as a fundraiser but, because of his involvement in deficit-cutting efforts like the Gang of Six, he is distrusted by some liberals.

“Progressives don’t think Social Security should ever be on the table,” Charles Chamberlain, executive director at Democracy for America, said when asked about Warner.

Kaine, the less controversial of the two, could be the sleeper candidate for veep that wins out.

“Obviously starts on the top of any list,” said Chris Lehane, a consultant who led Al Gore’s 2000 vice presidential search.

3. San Antonio Mayor, HUD nominee Julian Castro

Castro is generally seen as having massive potential but needing more time to establish his credentials.

“Castro is incredibly talented. Obviously, being Latino, he can help with the single most important dispositive voter cohort in the country,” Lehane said. “After being a cabinet secretary, assuming he performs well, he can check the box of having played at the national level.”

But others were not sure his role at HUD would help Castro establish himself on the big stage.

“I’m not convinced that running a large department like HUD is in any way, shape or form guaranteed to raise your profile or help you become the next vice president,” Manley said.

Still, Castro does have some bipartisan appeal.

“I’m not a Democrat strategist and for the life of me don’t understand their thinking in these things, but he’s, by all accounts, got a record in San Antonio worth looking at,” said Rep. Kevin Brady (R-Texas).

4. Sen. Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE (D-N.Y.) or Sen. Amy Klobuchar Amy KlobucharThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - White House moves closer to Pelosi on virus relief bill EPA delivers win for ethanol industry angered by waivers to refiners It's time for newspapers to stop endorsing presidential candidates MORE (D-Minn.)

The notion of two women on the ticket is getting a lot of discussion among Democrats, and both Gillibrand and Klobuchar are seen as strong contenders in that scenario.

Lehane noted a possible parallel: Bill Clinton in 1992 went for a “double down” strategy in picking Gore, another young politician from the South.

“The reinforced message was more important than geographic balance,” he said.

Under Democrats’ most optimistic scenarios, a historic two-woman ticket could drive the kind of turnout needed for a landslide victory.

5. Sen. Elizabeth Warren (D-Mass.)

Some progressives are wary about the Clintons’ ties to Wall Street, particularly deregulatory policies that they say helped lead to the financial meltdown in 2008. And while they'd happily get in line if Clinton were to select Sen. Elizabeth Warren (D-Mass.), most don't think that ticket is a realistic possibility.

Still, progressives are hopeful Clinton would pick someone from the "Warren wing" of the Democratic Party.

"The question becomes: Is she going to stay in the Wall Street wing – or move to the Warren wing of the Democratic Party?" said Chamberlain. "We’re concerned, in the past she’s been from the Wall Street wing of the party."

Meanwhile, former Pennsylvania Gov. Ed Rendell (D) pushed back against progressives who said that Clinton would need to pick someone who is more liberal on economic issues.

"The Elizabeth Warren wing of the party will not want a Republican to be elected," he said, "so they'll fall in line with Hillary."

6. New York Gov. Andrew Cuomo Andrew CuomoNYT editorial board remembers Ginsburg: She 'will forever have two legacies' New York to honor Ginsburg with statue in Brooklyn New York City bus driver knocked out by passenger he told to wear a mask MORE, Kentucky Gov. Steve Beshear or former Montana Gov. Brian Schweitzer

These three governors are wild cards that keep getting mentioned. Cuomo has been amassing legislative achievements and has Washington experience, but may be biding his time for his own presidential run.

Beshear is seen as strong and adding Southern regional balance even though no strategist sees Kentucky turning blue. Schweitzer has appeal to the center.