LAS VEGAS — When Beth Krerowicz began reviewing the platoon of Democratic presidential candidates, her first instinct was to back Joseph R. Biden Jr., whom she saw as the strongest opponent to President Trump.

But recently, Ms. Krerowicz, 58, began to have second thoughts. So this week she trekked to a community center not far from the Las Vegas Strip to watch Senator Elizabeth Warren of Massachusetts detail her plans for reshaping the economy. Ms. Krerowicz, an executive assistant who is between jobs, said that she was now leaning heavily toward Ms. Warren, and that Senator Kamala Harris of California was her second choice.

“I want someone who I know will stand up, that has a backbone,” Ms. Krerowicz said, suggesting that Ms. Warren and Ms. Harris could perhaps form a ticket. “They’re both very, very strong women. I would love to see them together, but I think Elizabeth has the experience.”

In the span of just a few weeks, voters like Ms. Krerowicz have pushed the race into a new, highly uncertain phase, propelling a pair of women toward the top of the Democratic pack at the expense of the onetime front-runners, Mr. Biden and Senator Bernie Sanders of Vermont.