Elizabeth Warren is suddenly having a harder time selling her big ideas in the Democratic primary. As president, she'd have an even tougher time convincing Congress — even if Democrats held both chambers.The big picture: A President Warren could immediately translate some of her progressive ideals into the policy of the land through executive order, but her best-known, most ambitious plans would have to go through Congress. And she’s virtually assured to lack the kind of majorities she’d need to pass them, according to Democratic staffers and former White House officials from both parties.Details: Even under the most optimistic scenario for Democrats — keeping the House and narrowly winning back the Senate from the Republicans — Warren wouldn’t have solid Democratic support for her biggest ideas, including the wealth tax, Medicare for All and the Green New Deal.Her college debt plan would likely get a major rewrite from fellow Democrats, in a best case scenario. They’re unlikely to go along with a full repeal of the GOP tax cut to set the stage for her corporate profits tax, according to Senate Democratic aides. And she’d have a solid wall of Republican opposition in the Senate, regardless of who wins the majority.Warren’s answer to that problem is to eliminate the filibuster so Democrats can pass legislation with a simple majority — but she’d have a hard time doing that since some Democrats, like Kyrsten Sinema, say they’d never support it.What they’re saying: “I don't think it's inappropriate for a candidate to campaign on her aspirations for the country, and to try to build support for a bigger vision than what is currently on offer,” said Cecilia Muñoz, who headed the White House Domestic Policy Council under Barack Obama and isn’t working for any of the presidential campaigns.“At the same time, I also think it's important to level with the country, because a big bold vision ultimately can take years to accomplish, and will require more than one election and much more than one leader.”Paul Winfree, who served as deputy director of the Domestic Policy Council under President Trump, said Warren would need Congress to pass “virtually everything of significance” on health care, banking, taxes, student loans and other issues.The other side: If Warren wins the White House, she could do some significant things through executive action — including tougher antitrust enforcement to try to break up the Big Tech companies, as Axios’ Margaret Harding McGill points out.Warren could appoint an aggressive Financial Stability Oversight Council to provide much tougher oversight of America’s biggest banks, Axios’ Felix Salmon notes, even if it couldn’t go quite as far as separating commercial banks from investment banks entirely. (Her proposed 21st Century Glass-Steagall Act would still require legislation.)On insulin, EpiPens, and the anti-inflammatory drug Humira, Warren says she’d be able to reduce the price of some costly drugs and delivery devices through executive actions. That could mean using existing legal authority to bypass or re-license their patents; and pursuing antitrust action against Humira manufacturer AbbVie.She couldn’t ban all fracking without approval from Congress, but she could ban it on federal lands, per Axios’ Amy Harder — and that could still have a significant impact, even though most oil and gas is produced on private and state lands. She could also reinstate environmental regulations Trump has repealed and make them even more aggressive. Warren could effectively decriminalize border crossings, not by changing the law but through instructions from the Justice Department telling federal prosecutors not to prosecute those crimes, Axios’ Stef Kight reports.The Warren campaign insists she could pass one of her most popular ideas — the wealth tax — through a budget reconciliation process that requires a simple majority vote in the Senate, and that she could use executive action to implement significant changes on health care, gun control and the environment."Elizabeth has a proven record of making our government work for every person, not just the wealthy and well-connected. She will use every tool available to do the same as president," said campaign spokesperson Saloni Sharma.Other variables may affect what Warren could or couldn’t do if she wins — like the state of the economy by 2021, and any unforeseen crises.The bottom line: Warren has built a campaign on her ability to lead through bold ideas, so it’s a good idea for supporters and critics alike to be clear-eyed about what she can actually get done.Go deeper:Reality check on Warren's biggest ideasElizabeth Warren on the issues, in under 500 wordsElizabeth Warren takes on private equityElizabeth Warren's dream health care world