The list of contenders Clinton is seriously weighing is believed to be fewer than five, several Democrats familiar with the process say, and all contenders are not on equal footing. She has a preferred candidate or two in mind, CNN has learned, but intends to keep her options open until Donald Trump reveals his selection.
Clinton is leaning toward announcing her decision immediately after the Republican convention ends on July 21. But aides say she is leaving open the possibility of waiting until Democrats are already gathering for their convention in Philadelphia the following week to disclose her selection, hoping to stoke drama and build interest in the Democratic ticket.
Cheryl Mills, a longtime confidant who is taking a lead role in the vetting process, walked into Clinton's residence in Northwest Washington on Thursday morning. James Hamilton, a veteran Washington lawyer who is overseeing the teams of lawyers researching the candidates, also was on hand. Clinton arrived less than an hour later and stayed until past sundown.
For nine hours Clinton's home was the site of constant activity, with more than half a dozen cars coming and going from the estate. All the cars were directed by Secret Service agents into the property through the protected driveway, so it was not clear who was inside the vehicles.
Mills and Clinton remained inside the entire time, joined off-and-on by teams of Democratic lawyers on the vetting team, each of whom are tasked with a particular assignment about one of the contenders. Only Clinton and a small clutch of advisers know which way she is truly leaning, but several Democrats familiar with the process describe this list of Democratic contenders:
The Virginia senator -- who Clinton has invited to campaign with her next Thursday in his critical battleground state -- is seen as a leading contender, several Democrats familiar with the process believe.
Kaine, a former governor and Democratic national chairman, is well-liked and seemingly checks Clinton's requirement for someone who could credibly step into the presidency. He is also fluent in Spanish, which the Clinton campaign believes could be a considerable asset attacking Trump in Spanish-language media.
He does not help Clinton shore up the liberal base of the party, but increasingly advisers believe that is less important, considering Bernie Sanders is set to endorse Clinton as early as next week.
He was fully vetted in 2008 by the Obama campaign. At the time, aides to Obama said nothing in Kaine's background raised red flags.
Kaine is also an accomplished fundraiser. He is headlining a conference call fundraiser for Clinton on Thursday, where donors in Paris, Moscow and Berlin will pay to listen to Kaine talk about the campaign.
The Massachusetts senator, once seen as an unlikely choice, is getting a more thorough review in the wake of rising populist sentiment in the electorate and Trump's harsh critique against Clinton on trade.
Warren's stock inside the campaign rose significantly after her appearance with Clinton last week in Cincinnati, according to aides. Noticing the response to their events was electric both inside and outside the room, aides believe Warren would provide an infusion of excitement to the campaign, especially with Sanders supporters.
Her vetting is the most complicated, people familiar with the process say, because she has been in the public eye far less than Kaine. And she also comes with a key issue: Questions remain about how Clinton and Warren would work together and whether Warren would overshadow her.
The Ohio senator is also a favorite of Clinton's, several Democrats close to the process believe, but the prospect of losing his Senate seat has placed him in a second tier. If he was selected, Republican Gov. John Kasich would appoint his replacement, who would remain until January 2019.
"Losing the Senate is too big of a risk," a longtime Clinton confidant told CNN, speaking on condition of anonymity to discuss the sensitive vice presidential selection. "Do you think Hillary is a risk-taker?"
Two members of President Barack Obama's cabinet are also in the mix: Agriculture Secretary Tom Vilsack, a former two-term governor of Iowa, and Tom Perez, the Labor secretary, whose biggest shortcoming is not holding elected federal or state office.
Vilsack would not upset the balance of power in the Senate and is seen as a steady choice, but Clinton is particularly fond of Perez for his willingness to throw a punch at Trump and his ability to relate to Latino voters.
Housing Secretary Julian Castro and Rep. Xavier Beccera, whose names were on a broader list, are no longer thought to be in serious contention, several Democrats close to the process believe.
While chemistry remains the most important ingredient in the decision, several Democrats close to the process say, Clinton is also keenly focused on how the political environment has changed since she started her search. The Brexit vote and Trump's relentless criticism of Clinton's position on trade agreements is something she is weighing, aides say.
Clinton's top aides say the process has moved past the time where deep dark horses and wild card picks are being considered. Yet she does have the luxury of waiting to see Trump's running mate before revealing her own because the Republican National Convention takes place first this year.
Clinton, according to aides, has been extraordinarily involved in the day-to-day aspects of the search and has received regular updates about the process. The meeting at Clinton's home on Thursday is an opportunity to study the complete picture of her final list of candidates and the last step before meeting face-to-face again with her top choices.
Clinton's top considerations, aides tell CNN, is someone who does not harm her chances against Trump, a partner who gets along well with her and is credible at stepping into the presidency should anything happen. Economic issues and an ability to raise money for the campaign are also key attributes.
While aides deny the Justice Department's decision not to indict Clinton for her use of a private email server does not factor in the decision, people close to Clinton believe the campaign must pick someone who Republicans will be unable to attack on questions of trust and honesty.