WASHINGTON — Hillary Clinton’s search for a running mate is moving into a more intense phase, according to several Democrats, as aides contact a pared-down pool of candidates to ask for reams of personal information and set up interviews with the presumptive Democratic nominee’s vetting team.

Those on the shortlist include Massachusetts Sen. Elizabeth Warren, a favorite of progressives who has emerged as a blistering critic of presumptive Republican nominee Donald Trump; Virginia Sen. Tim Kaine, a well-liked lawmaker from an important general election battleground state; and Housing and Urban Development Secretary Julian Castro of Texas, a rising star in the Democratic Party.

Vermont Sen. Bernie Sanders, Clinton’s toughest primary rival, is not on the shortlist of vice presidential candidates, according to one Democrat.

A small group of Clinton campaign confidants has been sifting through publicly available information about more than two dozen possible contenders for more than a month. But with Democratic primary voting wrapping up last week, the list has been culled significantly and the campaign has begun contacting those under consideration.

Several Democrats described Clinton’s vice presidential search process on the condition of anonymity, because they were not authorized by her campaign to publicly discuss it. Clinton campaign spokesman Brian Fallon would not comment.

Clinton was pressing forward on her search for a running mate as her GOP rival struggled with dismal fundraising and major questions about his campaign organization. Trump fired his campaign manager Monday and new fundraising data showed donors gave the businessman’s campaign just over $3 million last month.

If Clinton were to tap Warren as her No. 2, she would be choosing one of the Senate’s most outspoken liberals and a proponent of tough Wall Street regulation. She has long been viewed as a running mate who could help Clinton appeal to Sanders’ loyal supporters. She is also relishing taking on Trump, blasting the businessman in speeches and on Twitter as a “thin-skinned, racist bully.”