WASHINGTON — The fight over the future of the Democratic Party has been decided in the streets.

The swelling crowds at women's marches and the chanting airport cadres protesting President Donald Trump's new immigration plan have pushed the party to the left after years of mincing steps in that direction, most recently in the presidential contest.

After the Nov. 8 election, a convulsion began within Democratic ranks over how to rebuild from a stunning defeat. The options were to carve a path toward the white working class and suburban residents of middle America or toward the Obama coalition of urban, young and minority voters — or to sharpen a message that appealed to both.

Trump's presidency has shunted that debate aside, and the ensuing protests have defined the party as more liberal and more activist than it was even months ago. While some Democrats fear a fracturing of the party, others are convinced that opposition to the president will force unity on the factions.

“Trump has electrified the Democratic progressive base like nothing that has happened in the last 25 years,” said Ace Smith, a San Francisco-based Democratic strategist. “As depressing as what he's doing is, he's building an army that's going to fight him.”

Protesters have quickened the outrage metabolism among members of Congress, encouraged disruptive tactics, including the boycotts last week of Senate hearings on Trump's Cabinet nominees, and mostly ended the argument within the congressional caucuses over whether Democrats should work with Trump on occasion rather than universally oppose him.

Democrats now are experiencing both an ideological and a generational upending not unlike the one that decades ago gave many of today's Democratic leaders power they have been reluctant to cede. And it is one that some Democrats fear is too radical for the party as it looks toward 2018 Senate contests in swing states.

Howard Dean, the former party chairman, said Democrats must embrace the priorities of the street or risk demise.

“This is their Edmund Pettus Bridge, their Kent State,” he said, referring to sites of civil rights and anti-war struggles that propelled earlier activists into Democratic politics. “A party composed of all of the activists in the '60s and '70s — they changed the world, but they can't figure out how to hand this off to the next generation. The time for that is at hand.”

Still unclear, however, is whether the fiery passions of the protesters might end up consuming Democrats, much as a grass-roots rebellion on the right, which started a decade ago, caused warfare within the Republican Party.

Evidence of the potential for difficulty appeared last week on MoveOn.org, a liberal activist group.

“The public — which voted decisively against Trump — is demanding clear, principled, and total opposition to the Trump administration's extreme and unprecedented agenda,” the site said. “We hope Senate Democrats will hear that message — and quickly.”

Protesters angry that Democrats were voting for some of Trump's Cabinet nominees gathered at the homes of several party leaders, including Sens. Dianne Feinstein of California and Minority Leader Charles Schumer of New York.

How far the party goes to satisfy its newly vocal activists is particularly important to Democrats up for re-election in 2018 in states won by Trump, including Missouri, Montana, North Dakota and West Virginia. Each has a Democratic senator facing a potentially hostile electorate.

If the election focuses on national issues and positioning — a safe bet, given Trump's dominance of American politics — the leftward pull could hurt in those states, risking a Senate with fewer Democrats than the 48-seat minority the party has now.

Party leaders in the past have given red-state Democrats some license to move to the center in order to strengthen their standing at home. But with protesters demanding liberal purity, those officeholders are caught between activists who, if displeased, can hurt a candidate's support among the party faithful and more moderate voters who are needed to put them over the top.

To many Democrats, however, the desire for more vocal, passionate and leftward voices trumps that concern. Some of the party's potential 2020 presidential candidates have rushed to airports to protest the Trump immigration ban and to the stages of the women's march to rally supporters. On Jan. 31, when Trump nominated federal appeals court Judge Neil Gorsuch to the open Supreme Court seat, the announcement had barely occurred before Democratic emails flaying him appeared in inboxes.

The fervor at the protest rallies contrasts sharply with the reaction to Hillary Clinton through much of the 2016 campaign. She tabled blunt passion most of the time in favor of an approach more carefully calibrated to appeal to a wide range of voters, and her support was, in turn, more measured.

“We didn't have a message in 2016; we had a slogan,” Smith said. “Politics is never about how people are; it's about how people feel. The Hillary Clinton campaign was run by a bunch of people executing a paint-by-numbers strategy, and at the end, they didn't realize you don't win campaigns with an algorithm, but rather you win with passion and positions.”

Concern about those issues — and a surge in liberal views — had been growing for years, but debate over them was masked while President Barack Obama controlled the White House.

In 2000, a Pew Research survey found that 27 percent of Democrats called themselves liberal. By 2015, the percentage of self-described liberals had risen by 15 points. The biggest leap — 8 points — occurred between 2010 and 2015. (At the same time, Republicans have grown more conservative and the once-vibrant center has nearly vanished.)

Now, with Trump radicalizing Democrats, the two parties appear to be moving even farther apart. The demonstrations that have met the first two weeks of the Trump presidency may be only a harbinger of more intense struggles to come.

cathleen.decker@latimes.com