Mayor Muriel Bowser during a press conference in June 2015. (Bill O'Leary/The Washington Post)

Mayor Muriel E. Bowser is struggling to stand up to an aggressively liberal D.C. Council, which in the past six weeks alone has legalized assisted suicide in the nation’s capital and guaranteed new parents two months of paid time off.

A moderate Democrat whose roots are in the middle-class African American neighborhoods of Northeast Washington, Bowser increasingly finds herself at odds with a council infused with the leftist tendencies of the District’s millennial newcomers.

Heading into her third year in office, the mayor appears to be losing ground, sidelined as lawmakers advance a wish list of progressive policies without her cooperation, and sometimes despite her opposition.

The outcome could determine whether the city focuses on the national priorities of the political left — such as social-welfare programs and business regulation — or Bowser’s more conventional urban agenda of schools, crime and homelessness.

The latest display of the mayor’s predicament came last week, as council members gave preliminary approval to a $250 million-a-year program that will tax local businesses to provide private-sector workers with some of the nation’s most expansive family- and medical-leave benefits.

Citing its expense and disproportionate benefits to commuters from Virginia and Maryland, Bowser has repeatedly said she does not want to carry out the plan, which creates a new agency with a budget larger than that of the Department of Public Works.

But in a sign of her waning stature at city hall, it seems she will have little choice: The bill passed the council on its first reading with the supermajority necessary to override a veto.

“The dynamics are different, because the entire shape and political bent of the council have changed dramatically in just two election cycles,” said Adams Morgan neighborhood activist and former council candidate Bryan Weaver. “This really does sort of show it’s a new era. It will be tough for the mayor.”

Bowser’s defeat on paid leave could portend other struggles as she tries to counteract what some criticize as the free-spending ways of the council’s ascendant newer members.

Even in overwhelmingly Democratic big cities, mayors — whose duties include fighting crime and fiscal stewardship — sometimes act as a check on expensive or provocative impulses from legislators.

But Bowser will have few immediate tactical advantages as she tries to fulfill that role. When newly elected council members are seated next month, the bloc of four allies she relied on when she took office last year will shrink to one — Brandon T. Todd, her chosen successor for her old Ward 4 seat.

Some fear that the combination of a weakened mayor and an inexperienced, left-leaning council could mean trouble as the city seeks to preserve its hard-earned fiscal stability, with major investments in its transit system required over the next several years and questions about the future of federal funding under President-elect Donald Trump.

“With so many new people on the council, there’s a lot to learn about our unique fiscal situation,” said longtime District political activist Marie Drissel. At present, she added, the mayor “really doesn’t have the kind of support” she would need to put the brakes on fiscally reckless legislation.

City hall’s political equilibrium will be further unsettled by the return of former mayor Vincent Gray, who continues to command respect and influence despite a 2010 campaign-finance scandal that threatened to end his career.

Gray, a relatively centrist Democrat who was elected to the council in November to represent Ward 7, is a potential ally for the mayor from an ideological standpoint. But many question whether they can overcome the ill will that took root during the bruising 2014 mayoral campaign, which Gray lost to Bowser.

In an interview, Gray said he hoped to “work constructively” with the mayor despite their history, particularly on issues important to his constituents in the city’s poor easternmost neighborhoods. “I would hope that she and I can find common ground,” he said.

Gray will join a group of legislators whose relationships with the mayor and her staff have frayed, most recently in the conflict over family leave.

Council member David Grosso (I-At Large) attacked the mayor in a news release Dec. 5, the eve of the vote, saying she “cannot summon the courage to take a position” on paid leave herself and was instead working to gut the bill through Council member LaRuby May (D-Ward 8), one of her outgoing allies.

Such barbs are not particularly sharp by the standards set in recent showdowns at the John A. Wilson Building. In May, during a debate over city homeless shelters, a livid Bowser called Council Chairman Phil Mendelson (D) “a f------ liar” during a tense exchange that found its way into news reports.

“I don’t think that’s helpful,” said former council member Sekou Biddle. Especially when a politician does not command natural allegiance from colleagues, he said, it is crucial to leach the venom from personal relationships in city hall.

“It’s important to have a generosity of spirit,” Biddle said. “You have to, as much as possible, assume a positive intent from other people.”

Bowser declined an interview request. Her spokesman, Kevin Harris, said the mayor had been “incredibly effective” at moving her agenda through the council.

“One vote on one issue doesn’t change an extensive track record of passing legislation and budgets that have put the District in much better standing then when she took office,” Harris said.

Council member Elissa Silverman (I-At Large), a progressive who has served on the council since last year, said the mayor had done little to find common ground with opponents in areas where there is broader agreement, such as homelessness and education.

“My hope is the mayor looks to do more coalition-building with the council in the future,” Silverman said. “I think these past two years has been more of an all-or-nothing, ‘if you’re not with me you’re against me’ approach, which has limited our ability to work together in areas where we naturally agree.”

Silverman disputed what she said was a caricature of a runaway D.C. Council squandering taxpayer money. She said the council’s priorities have been similar to those in cities with similar demographics — such as San Francisco, Los Angeles and New York — and largely within the bounds of Democratic Party orthodoxy.

“Paid leave is in the Democratic Party platform,” she said. “We’re not being crazy leftist radicals here.”

There are many nuances to the political roles and power dynamics inside D.C. city hall. One of the District’s progressive coups this year — a plan to gradually raise the minimum wage to $15 — was led by Bowser, not the council.

Nor has the mayor been wholly ineffective in combating the leftward drift of city policy. Bowser has defended the District’s legalization of marijuana, enacted through a 2014 ballot measure by the same voters who put her in office.

But she drew the line early this year when the council sought to allow “clubs” where pot smokers could gather. Responding to her appeals, council members dropped the idea.

Even on the landmark family leave bill, the extent of victory for the progressive council members is not clear. The bill is scheduled for a final vote Dec. 20, leaving Bowser time to lobby for amendments before it passes.

The council has a vested interest in securing even grudging support from the mayor, who will be responsible for the program’s implementation.

Meanwhile, the breakneck pace of the city’s progressive agenda could be slowed somewhat by Mendelson, who is advocating a two-year moratorium on further workplace regulations to ease the effects of the paid-leave law’s tax on businesses.

Veteran D.C. activist and government watchdog Dorothy Brizill said the mayor and council could be forced to take up an all-consuming common cause if the District faces meddling from the Republican-dominated federal government that will assume power in about a month.

The new Congress and presidential administration — which have final authority over the city’s budget and legislation — could take aim at local laws and policies over which the District’s elected officials have no disagreement, such as gun control or health-care funding under the Affordable Care Act.

“No one knows what a Republican in the White House, in the form of Trump, is going to mean for the District of Columbia,” Brizill said. “I think the mayor and the council are going to have a number of issues on their hands.”