U.S. Senator Elizabeth Warren has considered the idea of serving as Hillary Clinton's running mate but sees obstacles to that choice as she prepares to endorse the presumptive Democratic presidential nominee, several people familiar with Warren's thinking told Reuters.

While her thinking could evolve, Warren has concerns about joining a Clinton ticket, including the question of whether running two women would give the Democrats the best shot at defeating Republican Donald Trump, one source said.

Advisers to Warren, a fiery critic of Wall Street and a popular figure among progressive Democrats, have been in close contact with Clinton's campaign team and the conversations have increased in frequency in recent weeks, the sources said.

Warren has signaled to people close to her that she is intrigued by the possibility of being Clinton's No. 2 but has not discussed the role with Clinton, 68, or anyone else from her campaign, the people said.