For years Nancy Pelosi, speaker of the US House of Representatives, was the most powerful woman in American politics, Congress's top Democrat squaring off against pugilist Republican president Donald Trump.

Today her fiercest battles -- over the scope of legislation, political ideology, and hot-button topics like Israel -- are not against an intransigent White House but rebellious progressives in her own caucus.

Nearly five months into Joe Biden's presidency, cracks have emerged in the Democratic edifice, and the 81-year-old Pelosi has her fingers in the dam.

Biden's transformational post-Covid agenda -- a once-in-a-generation investment in infrastructure, action on police reform, protections of voting rights, and fighting climate change -- faces Republican obstruction and a Washington system custom-built for gridlock.

But Pelosi must also balance her caucus's liberal flank eager to pull the party to the left, and centrists reluctant to support trillion-dollar programs.

Majority Leader Chuck Schumer faces similar challenges in the 50-50 Senate, where a single Democratic defector could derail Biden's wishlist.

Pelosi is being challenged to deploy her talents as a master strategist to appease her razor-thin majority in Congress and get big chunks of Biden's agenda into law.

Those in her party, and even some Republican rivals, say the California Democrat is clear-eyed about the battles ahead.

"This is her sweet spot," House Democrat Tim Ryan, who tried and failed to unseat then-House minority leader Pelosi in 2017, told AFP Thursday.

"She's masterful in the inside game of getting the votes, and I would never, never, never count her out to be able to do that."

Democrats hold a mere nine-seat majority in the 435-member House.

Many Washington watchers believe Republicans will reclaim the speaker's gavel in next year's midterm elections without even improving their 2020 vote total, with districts expected to be redrawn to their advantage.

Pelosi has immediate problems too. Progressives are threatening to sink a bipartisan infrastructure deal if it does not contain green energy proposals to address climate change.

A mini crisis also erupted last week when House Democrat Ilhan Omar made comments that several Democrats deemed anti-Semitic, anti-Israel, even anti-American.

Top House Republican Kevin McCarthy pounced, saying Democrats were "fighting among themselves" and that Biden's "own speaker of the House will not rebuke that."

But when Pelosi -- who agreed in 2018 to serve a maximum of four more years in the top job -- faced questions Thursday about whether she can unify her caucus to pass infrastructure, she did not hesitate.

"There aren't rifts. We're a Democratic Party, we're not a lockstep rubber stamp," she stressed.

"We are a party of ideas and dynamism, and we respect differences of opinion," she added. "But at the end of the day we know we will have unity to promote the well-being of America's working families."

'Crack the whip'

Paths to major legislative victories are far from assured, though, and a congressional clock is ticking.

Democratic strategist Michael Ceraso, who worked on progressive Senator Bernie Sanders's 2016 presidential campaign, said Pelosi and Schumer feel obligated to keep their flocks in line, while mollifying a rebellious left flank demanding big wins to tout during the midterms.

Progressives are frustrated, he told AFP, because "for five years, we kind of haven't seen any major piece of legislation passed that fits into the DNA of the progressive movement."

Ceraso said liberals are wondering how long before the legislative window shuts, "because we have an election season coming up (and) what are you going to be able to point to as a success?"

Kyle Kondik, a House election analyst at University of Virginia, notes that while Pelosi has few votes to spare, she maintains an advantage: her caucus is more "ideologically cohesive" than past Democratic majorities, which featured several conservatives.

Democratic divisions are putting Pelosi's legendary vote-counting skills to the test, according to Republican Tom Cole.

She "certainly knows how to crack the whip inside her own caucus, but you're going to have to use the whip more than you'd like to," he said.

"And I think that problem is getting more difficult going forward, not easier."

Pelosi was an effective nemesis to Trump. Now with Democrats running the White House and Congress, the pressure to perform is immense.

"Everybody is more effective when you've got a foil on the other side," Cole said. "It's very tough when you control everything."

Pelosi's admirers maintain however that these divided times call for a unifier rather than a brawler -- and Pelosi fits the bill better than just about anyone in Washington.

"I have seen no slippage in Nancy Pelosi," said Gerry Connolly, a seven-term Democrat. "None."