Democratic Party's presumptive presidential nominee Hillary Clinton has narrowed down her choices for a running mate, officials said. Clinton has not yet conducted formal interviews, but has devoted hours studying the records and backgrounds of several Democrats on a list that includes Senator Tim Kaine of Virginia, Senator Elizabeth Warren of Massachusetts and Housing and Urban Development Secretary Julian Castro of Texas, the party officials told CNN on Tuesday. The list also includes Labour Secretary Tom Perez, Senator Sherrod Brown of Ohio and Representative Xavier Beccera of California. However, the list does not include Senator Bernie Sanders of Vermont, her primary rival who is yet to endorse her candidacy but has pledged to help defeat Republican Party's Donald Trump. He was not expecting to be considered, Clinton's aides said. John Podesta, chairman of the Clinton campaign and a trusted confidante, is leading the effort, along with Cheryl Mills, the former secretary of state's longtime adviser and lawyer. Clinton has repeatedly said in interviews, her top consideration is someone who would be able to step into the presidency should anything happen to her. "I want to be sure that whoever I pick could be president immediately if something were to happen," Clinton told CNN earlier this month, adding "That's the most important qualification".