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.
Neither woman is yet in a position to take control of the race. Mr. Biden and Mr. Sanders retain considerable advantages, most notably the backing they enjoy among working-class voters and their significant financial reserves. Mr. Biden, 76, is still the clear front-runner, and both he and Mr. Sanders, 77, are not likely to see their support simply melt away.
But recent polls, and interviews with voters in the early primary and caucus states over the last week, found that Ms. Harris and Ms. Warren had plainly broken through, drawing on a deep hunger within the Democratic electorate for big ideas and groundbreaking female leadership.
Together, the two senators have functioned as something of a political pincer, squeezing Mr. Biden and Mr. Sanders from the left and the relative center, and endangering any hope of an easy march to victory for either man.
After Ms. Harris’s searing clash with Mr. Biden over his record on race in the first primary debate, she claimed for herself a chunk of Mr. Biden’s formidable support among African-Americans and white liberals. And Ms. Warren, with her relentless focus on economic inequality and political corruption, and her full portfolio of progressive policy proposals, has cut deeply into the coalition Mr. Sanders built during his 2016 campaign.
Their performances in the first debate lifted both women, with Ms. Harris making an especially sharp ascent after confronting Mr. Biden — a showdown that convinced some voters she had the toughness necessary for the general election.
”I just think she has the moxie to bring it to the Donald,” said Claire Haws, a stay-at-home mother from Des Moines, who contributed to Ms. Harris’s campaign after watching her in the debate.
[Which Democrats are leading the 2020 presidential race this week?]
Yet both women still face a complex path forward, one complicated by the enduring strengths of Mr. Biden and Mr. Sanders, and perhaps most of all by each other. While they have distinct political bases, Ms. Warren and Ms. Harris are both relying on the support of women and educated liberals to propel their candidacies. That strategy also means they are dividing up powerful constituencies that could in theory make one of them a dominant figure.
“They’re in the same lane right now, and it’s getting pretty crowded,” Patty Judge, Iowa’s former agriculture secretary, said.
The two women also still face competition for their core supporters from other dogged candidates reaching for the mantle of change, like Mayor Pete Buttigieg of South Bend, Ind., and Senators Cory Booker of New Jersey and Amy Klobuchar of Minnesota.
Former Senator Harry Reid of Nevada, the longtime Democratic leader in that chamber, said Mr. Biden still had an “early advantage.” Though he is neutral in Nevada’s early caucus, Mr. Reid, who worked closely with Mr. Biden for 30 years, said the former vice president should not be underestimated.
“Joe Biden has a great deal of good will that’s been created over the years that he’s been in politics,” Mr. Reid said, pointing to Mr. Biden’s tragedy-inflected biography and expansive network of political friendships as foundations for his candidacy. “Here’s a man whose life story is something you can’t ignore.”
But Mr. Reid also praised the two women who are surging in the race, calling Ms. Harris a candidate “you have to be impressed with” and describing Ms. Warren’s indignant economic message as a good match for his own state, which was ravaged by the Great Recession. “Here in Nevada,” he said, “it resonates quite well.”
Ms. Harris and Ms. Warren have been eyeing each other warily for months, anticipating a tug of war for Democratic voters who are eager for a fresher-faced champion than Mr. Biden or Mr. Sanders. A new poll in Iowa showed that voters who list either woman as their top choice list the other as their most common second-choice preference. Ms. Harris already appears to have cut into Ms. Warren’s support from liberals, and she is also improving with moderates and African-Americans — a combination of constituencies that prove pivotal in Democratic primaries.
[Sign up for our politics newsletter and join the conversation about the 2020 presidential race.]
Some in Ms. Warren’s camp have long seen Ms. Harris as a serious threat, but have also questioned whether Ms. Harris can sustain the energy she has generated with set-piece moments like her takedown of Mr. Biden. Ms. Harris has repeatedly bobbled policy issues, most recently after her debate confrontation last week with the former vice president over school busing, when she was quizzed on her own views on busing and struggled to differentiate her position from that of Mr. Biden.
Ms. Harris’s advisers, in turn, are convinced that Ms. Warren will ultimately suffer for her fixation on her policy agenda and relative lack of interest in going after Mr. Trump directly. For all of the enthusiasm over Ms. Warren’s churn of ideas, many Democrats remain deeply concerned about her readiness to face Mr. Trump in the general election.
The two women’s contrasting approaches to the race were on vivid display over the last week, as Ms. Harris carried her momentum into Iowa and Ms. Warren pressed her advantage in Nevada. Ms. Harris, tapping into the excitement surrounding her debate performance, debuted a new line of attack against Mr. Trump, encouraging Democrats to see her as a prizefighter pursuing a president they loathe.
”We have a predator living in the White House,” Ms. Harris told Democrats at an afternoon picnic outside Des Moines.
It is that sort of pugilism — wielded to great effect against Mr. Biden in the debate — that has endeared Ms. Harris to Democratic activists in Iowa and around the country. It has filled them with an almost giddy anticipation to watch Ms. Harris humiliate Mr. Trump on the debate stage next year.
“You dominate a debate stage and people are going to see you in a different light,” said Jennifer Palmieri, a top aide to Hillary Clinton in 2016. “Women winning makes other women more comfortable.”
Ms. Haws, the Des Moines voter, said she was determined to caucus for a woman and had also considered Ms. Warren. But she worried that Ms. Warren could represent an easier target for the president.
”The Pocahontas thing gives me pause; I’ll be honest, it makes me nervous,” Ms. Haws said of Mr. Trump’s penchant for ridiculing Ms. Warren about her claims of Native American ancestry.
For other Iowa Democrats, their preference for Ms. Harris owes less to an unease about Ms. Warren than an inclination for a younger candidate who offers racial diversity.
”When you can have a person of color that this state votes for, that says something,” said Mary Kay Shanley, a West Des Moines writer. She pointed to the symbolism of a heavily white state propelling the daughter of Jamaican and Indian immigrants to the presidency.
Even seven months before the vote, the choice between Ms. Warren and Ms. Harris is already an agonizing one for some Iowa Democrats. “They’re tied in my head,” said Claire Celsi, a state senator. “If I had to choose today, I’m not sure what I’d do.”
In Nevada, Ms. Warren largely ignored Mr. Trump, but showed the potency of her own approach on the eve of the July 4 holiday, when she visited Las Vegas and Reno to rail against the government for having abandoned the state to the greed of mortgage lenders. Her teeming crowds, dotted with neon-streaked hair and the occasional cowboy hat, roared with approval as she ticked off pledges to enact substantial government initiatives in housing and education, exclaiming repeatedly, “And I’m not through yet!”
“If you want to make change,” Ms. Warren said in Reno, “the way you have to make it happen is you’ve got to have a plan.”
Ms. Warren, with her torrent of ambitious ideas and more agile campaign, appears to have already overtaken Mr. Sanders in Nevada, according to Democratic leaders in the state. But Nevada progressives say Mr. Sanders’s lingering presence could keep her from achieving a clear lead.
Nelson Beltran, 29, a voter in Las Vegas, said he had supported Mr. Sanders in the 2016 caucuses, but had now shifted toward Ms. Warren because of how she confronted Wells Fargo executives for allegations of massive fraud.
“Seeing her go after bankers spoke to me a lot,” said Mr. Beltran, who works in film production. “Bernie’s still in there, but I’m more partial toward Warren this time around.”
Chris Giunchigliani, a former member of the Clark County Commission who ran for governor last year as a progressive insurgent, said after attending Ms. Warren’s event in Las Vegas that she was inclined to back the Massachusetts populist. She called Ms. Harris intriguing, but said she had concerns about her record on criminal justice.
Ms. Giunchigliani warned that there could still be resistance in the state to nominating a woman — a subject she said she had discussed with Ms. Warren privately. Nevada, she cautioned, could still be “very sexist” where politics is concerned.
“She’s smart as a whip, but as a woman, that doesn’t always matter,” Ms. Giunchigliani said of Ms. Warren, adding hopefully, “I think 2020 is going to be a different type of campaign here.”