Sen. Bernie Sanders and other prominent liberals are calling Donald Trump’s plan to rebuild America a “scam” and a “trap” that liberals should reject.

The more hostile tone toward Trump’s goal to repair the nation’s roads, bridges, ports and other public works has emerged as details of the president-elect’s plan seep out. The incoming administration hasn’t fully laid out all its proposals, but they seem to rely more heavily on tax incentives instead of direct government spending as preferred by Democrats.

In a series of tweets Monday night, Sanders blasted Trump’s apparent plan as “nothing more than corporate welfare.” The former Democratic presidential contender also penned an articlecalling for a different approach.

After Trump was elected, he vowed to “rebuild our highways, bridges, tunnels, airports, schools, hospitals” and employ millions of Americans in the process. One of his top advisers, Steven Bannon, said in an interview with The Hollywood Reporter that he was pushing for a $1 trillion infrastructure-rebuilding plan to help prop up America’s middle class.

Read: With negative interest rates, Bannon says ‘rebuild everything’

Democratic leaders including Sanders initially indicated a willingness to work with Trump to achieve their long-sought goal of more federal spending on public works. President Obama has pushed for higher spending for years, though Republicans controlling Congress have resisted.

Yet Democrats have quickly become more guarded in their public comments. The new Senate minority leader, Charles Schumer of New York, said he’s willing to support an infrastructure plan but “it has to have certain things for us to support it. It can’t be just tax credits. That won’t be enough.”

Ron Klain, a former chief of staff for Vice President Joe Biden and ex-Vice President Al Gore, said it would be a grave mistake for Democrats to back Trump’s plan.

“Don’t do it. It’s a trap,” he wrote in a column in the Washington Post.

Klain argued that private investors would benefit most and that badly needed but less sexy projects such as repair of water systems like the one in Flint, Mich. would get ignored.

Noted economist and New York Times columnist Paul Krugman, a staunch liberal influential among Democrats, has made the same argument. An ardent Trump foe, he called the president-elect’s plan “basically fraudulent” and an “exercise in “crony capitalism.”

The full extent of Trump’s plan probably won’t be unveiled until 2017. As with many of his promises, Trump has to thread a needle to fulfill them. While some Republicans support more spending on public works, budget hawks worry it would expand the nation’s deficit. The U.S. ran a shortfall of about $600 billion in the fiscal 2016 to push the national debt up to $20 trillion.

Some of Trump’s advisers are also less enamored of a broad infrastructure spending bill. Economist Judy Shelton said in a recent interview she prefers a lower corporate tax rate as a means to encourage spending instead of “stimulus that’s just more government spending.”

Trump could try to sweeten the pot by backing a partial increase in federal spending and perhaps set up a so-called infrastructure bank — an idea backed by Democrats — that would help fund projects with a mixture of investor and government money.

Even that might not be enough. A large swath of the Democratic party and its supporters believe as Klain does that it’s a fool’s errand to agree to any deals with Trump that boosts his political success and thus popularity with the American public.

Trump might hold one wild card, though.

Some 25 Democratic senators are up for reelection in 2018 and one-third represent states that voted handily for the New York businessman. Democrats could face the possibility of Republicans winning a veto-proof majority in the Senate for the first time since 1908 if Trump and his policies turn out to be popular.