Democratic leader Nancy Pelosi talked for eight hours on Wednesday in an attempt to push for vote on extending protections for the "Dreamers" - immigrants brought to the US as children.

Pelosi spent much of the address reading personal letters from the young people whose temporary protection from deportation is set to expire next month.

The House minority leader quoted from the Bible and Pope Francis, as Democrats took turns sitting behind her in support. The House Historian later confirmed it was the longest continuous speech ever given in the chamber.

"You see, these people are being deported," Pelosi said, at around the sixth hour of her speech. "We can do something today to at least make whole the children."

The speech had little impact on Republicans, who have not agreed to a vote.

However, the speech was also aimed at the liberal wing of Pelosi's own party, who were seething on Wednesday as Democrats in the Senate cut a budget deal that could quickly steal the momentum behind the effort to resolve the uncertain future of the Dreamers.

Immigration activists in the capital on Wednesday threatened political retribution against the Democrats for abandoning the strategy of demanding a budget deal is paired with an immigration deal, threats that expose a schism within a Party.

The activists called out Pelosi and Senate Minority Leader Chuck Schumer by name.

Some scoffed at Pelosi's speech, intended to elicit a promise from House Speaker Paul Ryan to allow a vote on subsequent legislation to protect the younger immigrants. Ryan's promise, activists noted, was far from a guarantee.

"What are they thinking? They're giving up their leverage," said a frustrated Angel Padilla, policy director for the liberal group Indivisible. "All of these votes will matter come November."

Pelosi started her remarks at about 10am and yielded the floor at 6.11pm.

By the end of the marathon, the clearest signs of weariness were an occasional quiver in her voice, a stumble over her words and a case of the sniffles. At one point she interrupted herself to read a note from the House historian alerting her that she had delivered the longest continuous speech, besting Republican Champ Clark's five-hour, 15-minute stem-winder about tariff reform in 1909.

Pelosi received a standing ovation from the Democrats at the end. "Honour the House of Representative and give us a chance to have a vote on the floor," she said before leaving.

At issue is the fate of roughly 1.8 million immigrants brought to the country as children and living here illegally. Many of the so-called "Dreamers" could lose protection from deportation - granted by the Obama administration in 2014 and rescinded by Donald Trump last fall - in the coming weeks.