Warren Is Preparing for 2020. So Are Biden, Booker, Harris and Sanders.

RENO, Nev. — During a campaign-style tour of the West late last month, Senator Elizabeth Warren did not announce she was running for president. But in private events and public speeches, her message about 2020 was as clear as it was rousing.

In Salt Lake City, Ms. Warren urged Democrats to turn out in force for the midterm elections to build momentum for the next presidential race, and in Denver, she told a meeting of state legislators and trial lawyers that she wanted to be a tribune for lower-income Americans, according to people who attended the events. And in a speech to the Nevada Democratic Party in Reno, Ms. Warren said Democrats must do more than “drive Donald Trump and his enablers out of power.”

“I want a party strong enough to take on the hard job of cleaning up the mess they’ll leave behind once they are gone,” Ms. Warren declared, all but volunteering for the task.

Before the trip and since, Ms. Warren and her emissaries have been reaching out to key Democratic officeholders in Iowa, Nevada and South Carolina — three states early in the presidential primary calendar — making introductions and offering help in the midterm campaign. Altogether, her moves are among the most assertive steps taken by any Democrat to prepare for 2020.

Ms. Warren, 69, now leads a small advance guard of Democrats who appear to be moving deliberately toward challenging President Trump. Former Vice President Joseph R. Biden Jr., wielding a political network cultivated over decades, has been reasserting himself as a party leader, while Senators Cory Booker of New Jersey and Kamala Harris of California have emerged as fresher-faced messengers for the midterms. And Senator Bernie Sanders of Vermont, the runner-up in the 2016 primaries, has been acting like a candidate as he considers another race.

All five have been traveling the country, raising money for Democrats and gauging the appeal of their personalities and favorite themes. As a group, they are a strikingly heterogeneous array of rivals for Mr. Trump, embodying the Democratic Party’s options for defining itself: They are distinguished by gender and race, span three decades in age and traverse the ideological and tonal spectrum between combative Democratic socialism and consensus-minded incrementalism.

Yet absent, at least so far, is either an obvious political phenom like former President Barack Obama or an establishment-backed juggernaut in the mold of Hillary Clinton. Unlike the last few Democratic primaries, the unsettled race evokes the sprawling nomination fights of earlier decades — lacking a dominant figure and seemingly inviting new leaders to rise.

“The opportunity for somebody to emerge and catch a wave hasn’t been this high since 1976,” said Anita Dunn, a veteran Democratic strategist, referring to another unpredictable primary featuring a multitude of candidates and a party wrestling with its identity.

Interviews with about four dozen lawmakers, consultants and party leaders revealed a mood of emphatic uncertainty: Senior Democrats see their party in a historically volatile state, and they are wary of attempting another Clinton-style coronation. But many Democrats believe the party’s turn left, combined with the rising fury of progressive women and the grass-roots appetite for a political brawler, have created an especially inviting environment for Ms. Warren.

[Here’s what’s coming up next on the primary calendar.]

On her Western swing, Ms. Warren sought to strike a unifying chord. At a tapas restaurant in Salt Lake City, she said Democrats had to close ranks in 2018 in order to recapture the White House.

“She was saying, ‘Look, we all have to come together,’” recalled Jackie Biskupski, Salt Lake City’s Democratic mayor, who attended the event. She paraphrased Ms. Warren’s exhortation: “We need to show this year that change is what the people want.”

Perhaps most appealing to Democratic leaders, Ms. Warren might please their activist base while staving off a candidate they fear would lose the general election.

A candidate such as Mr. Sanders.

The 76-year-old Democratic socialist looms over the 2020 race, boasting an unmatched following among activists and a proven ability to raise millions of dollars online. Having pushed policies like single-payer health care and free public college tuition into the Democratic mainstream, Mr. Sanders could be a powerful competitor for the nomination — and a daunting obstacle to Ms. Warren and other economic populists.

But for all the evident support for Mr. Sanders’s policy ideas, many in the party are skeptical that a fiery activist in his eighth decade would have broad enough appeal to oust Mr. Trump.

Mr. Sanders’s generational peer, Mr. Biden, 75, is preparing to test a contrasting message this fall, with plans to campaign up to four days a week after Labor Day, people familiar with his strategy said. In his speeches so far, Mr. Biden has struck a gentler chord than Mr. Sanders and Ms. Warren, delivering paeans to bipartisanship and beckoning Democrats to rise above Mr. Trump’s demagogic taunts.

And Mr. Biden, who has run for president twice before, has been seeking out a younger cohort of Democrats: During a trip to New Orleans in June he arranged a visit with Representative Cedric Richmond, the 44-year-old chairman of the Congressional Black Caucus, and this month he will travel to Arizona to help Representative Kyrsten Sinema, the Democrats’ 41-year-old Senate candidate, and to address a gathering of the Latino advocacy group Lulac.

But Mr. Biden’s most important step so far has been to help install a close ally, James Smith, as the Democratic nominee for governor in the early primary state of South Carolina. Steve Benjamin, the Democratic mayor of Columbia, South Carolina’s capital, said Mr. Biden enjoys a close bond with party leaders there.

“The vice president has a lot of long and deep relationships in South Carolina, and they’re substantive relationships,” said Mr. Benjamin, acknowledging Mr. Biden’s staff had called him recently.

But Mr. Benjamin said Mr. Biden was not the only contender getting in touch: Aides to Ms. Warren had also contacted him, he said, to arrange a call with her about a visit he made to the Mexican border.

Mr. Biden is not committed to running and recognizes that the party is drifting from his institutionalist style and relative moderation, people who have spoken to him said. In what some Democrats have taken as a sign of hesitation, Mr. Biden has not attempted to deter other competitors: Meeting recently with two other possible candidates, Mayor Eric M. Garcetti of Los Angeles and Gov. Steve Bullock of Montana, Mr. Biden advised both men to make their 2020 decisions without concern for his own, people briefed on the conversations said.

Ms. Harris and Mr. Booker have shown no such ambivalence. The two younger Democrats in the 2020 vanguard have been campaigning for fellow senators, and at times intervening in contested primaries to support like-minded candidates. Both also sit on the Senate Judiciary Committee — the panel that will scrutinize Judge Brett M. Kavanaugh, Mr. Trump’s Supreme Court nominee, in the coming months — giving them a high-profile platform to challenge the president.

Ms. Harris, 53, has been focused on backing Democratic women and candidates of color, endorsing over a dozen around the country. And at an April gathering of The Links, a service organization of African-American women, Ms. Harris met with Deidre DeJear, a black woman who is the Democratic nominee for secretary of state in Iowa.

In her travels, Ms. Harris offers herself as the herald of a rising, diverse generation of Americans. An unrelenting critic of the president, she is said to have grown more interested in the 2020 race as the midterm campaign has unfolded and Democratic women have mobilized to repudiate Mr. Trump’s party.

Some Democrats delight at the contrast between her and the 72-year-old president.

“I think this world needs a black woman as president,” said John Bowman, a former Missouri state lawmaker, after hearing Ms. Harris address an N.A.A.C.P. gathering in St. Louis last month.

Mr. Booker, 49, has campaigned with a message of uplift, aiming to show that he can win over voters in red states as an African-American liberal from the Northeast. After stumping in Alabama for Senator Doug Jones last year, Mr. Booker is said to be considering a fall swing through the Deep South, a conservative region where black voters have helped settle the last two Democratic nominations.

He is also in regular contact with early-state Democrats like the influential New Hampshire lobbyist Jim Demers. An early backer of Mr. Obama, Mr. Demers said in an interview he had not planned to pick a candidate early this time. But much as he swooned in 2006, Mr. Demers said he could not resist Mr. Booker and had recently told the New Jersey senator he would support him if he ran.

Democrats expect many more candidates to emerge, unless the midterms become a chilling disappointment. Several ambitious Democrats who have been more muted so far are poised to step up their activity: Deval Patrick, a former Massachusetts governor advised by some of Mr. Obama’s close associates, is expected to create a political committee this year and is rejoining the fray this month, addressing the N.A.A.C.P. convention in San Antonio on Monday and appearing over the weekend with Colin Allred, a Dallas-area congressional candidate.