A new generation of Democrats is using far-reaching policy ideas and a brash social media presence to upend the party — pushing it to the left on divisive issues such as health care and climate change while it charts a path aimed at taking the White House in 2020.

But the liberal shift, and the lawmakers driving it, are also creating challenges for Democrats in more-conservative areas, and they are giving President Trump and congressional Republicans fresh opportunities for political attacks. The GOP has been particularly focused in recent days on hammering Democrats over draft details of a broad “Green New Deal” proposal, even if most Democrats have not directly endorsed the fine print.

 The party’s increasingly liberal bent is also creating dilemmas for House Speaker Nancy Pelosi (D-Calif.), who is trying to manage a band of outspoken new liberal members while also staying attuned to the needs of moderates hailing from swing districts that could be key to Democrats retaining the House majority in 2020. 

“We won the House through the middle,” said Rep. Josh Gottheimer (D-N.J.), who co-leads the Problem Solvers Caucus. “Our party has to be open and recognize that. And if we don’t and insist that everyone takes a hard line view on everything, (a) I don’t think that’s going to attract votes in the next election, and (b) it puts our majority at risk.” 

Sen. Joe Manchin III (D-W.Va.) said of his party’s shift to the left: “It makes it more difficult in more-rural areas like mine. Absolutely makes it more difficult.” 

But many lawmakers — including 2020 presidential candidates — argue that a general push leftward is long overdue and reflects the broad popularity of many of their positions, from universal health coverage to taxing billionaires at higher rates.

“You can feel it now. In Congress, it has started to shift. It has started to shift. We’re not there yet. We’re not all the way there, but it has started to shift,” Sen. Elizabeth Warren (D-Mass.) said at a presidential campaign event Sunday night in Davenport, Iowa, when discussing her support for the Green New Deal. “We’ve got new energy in this. We’ve got new blood in this. We’ve got new ideas in this. No one is saying there’s a single silver bullet that’s going to fix the whole problem. What’s happening is people are coming up with lots of pieces.”

Trump and the Republicans see the shift — and the Democratic division — as an opportunity to portray their opponents as “radical” and out of touch with mainstream America. The attacks have been fueled further by actions on the part of some Democrats, including remarks from Sen. Kamala D. Harris (D-Calif.) supporting the elimination of private insurance as part of a single-payer health-care system and a document from the office of Rep. Alexandria Ocasio-Cortez (D-N.Y.) outlining details of the Green New Deal that her office has since disavowed.

At a campaign rally Monday night in El Paso, Trump mocked the Green New Deal as a “high school term paper that got a low mark” and argued to his crowd of supporters that it would “shut down American energy”  and “would shut down a little thing called air travel.” 

“It all has to do with 2020 and the election,” Trump added. “But I really don’t like their policy of taking away your car, of taking away your airplane flights, of ‘Let’s hop a train to California.’ ” 

Trump continued in that vein during a meeting with Cabinet officials Tuesday. “There’s tremendous and quite unusual attitude on the other side. There’s a lot of anger. And they’re slipping extremely far left,” Trump said. “We don’t want that to happen to our country.”

On Tuesday, Senate Majority Leader Mitch McConnell (R-Ky.) said he plans a floor vote on a Green New Deal resolution that has been endorsed by some 2020 contenders, a bid to force Democrats to adopt what Republicans contend is an unpopular position.

“He’s trying to bully the party, and he’s banking on people not being courageous,” Ocasio-Cortez said Tuesday of McConnell’s plans. “I think people should call his bluff.”

Some potential 2020 contenders, however, are treading carefully, particularly after aides to Ocasio-Cortez, the plan’s most prominent advocate, published and distributed a list of “frequently asked questions” that included details not included in the resolution itself, such as ensuring economic security for those “unwilling to work.” Her office later said it had inadvertently published an early version. 

Sen. Sherrod Brown (D-Ohio), who is considering a 2020 presidential bid from a state that has been shifting to the right, said Tuesday that elected officials ought to address climate change aggressively but stopped short of endorsing the specific resolution backed by Ocasio-Cortez and others.

“I’m not going to get in the position of every time somebody has a really good idea or a big idea that I have to talk in great detail about my position on it,” Brown said at a breakfast with reporters hosted by the Christian Science Monitor. “I know that the easy thing to do is say, ‘Yes, yes, yes, yes, yes, yes, yes, yes, yes.’ ” 

At her appearance in Davenport, Warren gave a lengthy answer when asked whether she supports the Green New Deal resolution, saying that she’s gratified to see so much attention focused on climate change but steered clear of some of the specifics in the proposal. 

And at a Monday night town hall meeting in Iowa City — where she was introduced as “the most progressive candidate” in the race — Rep. Tulsi Gabbard (D-Hawaii) said in an interview that the Green New Deal resolution did not have enough specifics for her. 

“There’s a lot of things that are vague and can be open to misinterpretation or lead to unintended consequences,” Gabbard said. “Fracking [hydraulic fracturing] and nuclear power are two things that are not mentioned in that resolution at all. The waste that is produced by nuclear power is waste that we’re going to have to deal with forever.” 

The Green New Deal is just one of several issues where candidates are describing one thing while activists are demanding another, even when using the same language.

With the exception of Sen. Amy Klobuchar (D-Minn.), for example, every Democratic senator running for president has endorsed “Medicare-for-all” legislation, which if implemented would phase in a national health plan with only limited roles for private insurers. 

But each Democrat has described the goal differently. Sen. Cory Booker (D-N.J.) said in his weekend stops across Iowa that he backed a “Medicare-for-all option,” describing a government program into which people could buy while leaving private insurance in place. 

Six of the seven senators who are seeking the Democratic nomination or seriously considering bids have signed on to a Green New Deal resolution from Sen. Edward J. Markey (D-Mass.). Five of the seven put their names on a Medicare-for-all proposal from the last Congress that would phase out private health insurance. 

Meanwhile, Klobuchar, who announced her presidential candidacy this week, distanced herself from the details of the Green New Deal proposal circulated by Ocasio-Cortez’s office. She said Tuesday on Fox News that she would vote for the climate change resolution but that the situation would be different “once it got down to the nitty-gritty of . . . actual legislation.”

Democratic pollster Celinda Lake said the Green New Deal and Medicare-for-all plans are popular concepts among Democratic voters, but she said they are not well-defined among the broader public. 

“What Democrats should be doing is laying out some core principles about what should be in the bills,” Lake said. “People like them a lot but don’t know what’s in them.” 

In the House, Pelosi and her lieutenants have tried to give a nod to progressives without letting them control the agenda. Though popular among the 2020 field, most of the proposals are unlikely to get a floor vote in the House since Democratic leaders think they would be likely to backfire on the party. 

Indeed, the Medicare-for-all plan endorsed by liberals is not even expected to get a hearing, much less a committee vote, in the pivotal House Energy and Commerce Committee, which technically has jurisdiction over health care. Rather, the proposal has been steered to the second-tier House Rules and Budget panels, which will vet the proposals without voting on them. 

And at times, Pelosi and her allies have been forced to play damage control to keep progressive members — particularly freshmen — in line.

 When Rep. Rashida Tlaib (D-Mich.) told a crowd of supporters last month that Democrats would “impeach the motherf-----,” referring to Trump, Rep. Debbie Dingell (D-Mich.), a close ally of Tlaib’s and Pelosi’s, pulled Tlaib aside and counseled her to be careful. When officials at the American Israel Public Affairs Committee and other Jewish leaders angrily called leadership offices about remarks made by Rep. Ilhan Omar (D-Minn.) that were seen as insulting to Jews and Israel, Pelosi crafted a rare public rebuke of the freshman, calling on her to apologize for her “anti-Semitic tropes.”

On the policy front, particularly on health care, Pelosi is focusing the caucus’s attention on improving the Affordable Care Act, also known as Obamacare, which was her chief legislative accomplishment during her first term as speaker. 

“She lets enough air out of the balloon before it pops,” said one senior Democratic aide of Pelosi’s approach to progressives, throwing them something to keep them happy without allowing their proposal to take over the House agenda. 

The Green New Deal has received little consideration from most senior House Democrats, who saw their centrist Blue Dog Coalition go nearly extinct a decade ago after Pelosi and other leaders forced moderates to vote on cap-and-trade legislation to manage industrial emissions of greenhouse gases. While more than 65 progressive House Democrats have signed onto the Green New Deal, many other Democrats privately scoff at the resolution. 

“I think it’s great when new members come in and have ideas and give speeches,” said Sen. Christopher A. Coons (D-Del.). “But I’m struck at how often I’m being asked by the press about a gauzy resolution that has no details to it.” 

Sean Sullivan in Washington and David Weigel in Iowa City contributed to this report.