House Minority Leader Nancy Pelosi (D-Calif.) speaks about the Republican tax bill as Senate Minority Leader Charles E. Schumer (D-N.Y.) listens at the U.S. Capitol. (Jim Lo Scalzo/EPA-EFE/Shutterstock)

In just six seconds, the new ad from the Democratic Senatorial Campaign Committee previews an attack that will have millions of dollars behind it in 2018.

“The Republican tax scheme gives huge breaks to corporations but raises taxes on middle class families,” says a narrator, as a woman checks her mailbox and recoils.

Democrats, routed but unified against the tax bill, plan to make it the centerpiece of a midterm campaign — one that may play out in a growing economy where the worst predictions about the tax cuts fall flat. Republicans, who once hoped that Democrats would feel pressured to back the bill, now suggest that voters will learn that the Democrats misled them.

“If we can’t sell this to the American people, we ought to go into another line of work,” Senate Majority Leader Mitch McConnell (R-Ky.) said after the bill’s passage.

Republicans start that sales pitch from a surprisingly weak position. After months of rallies, presidential salesmanship, TV ads and even a Republican National Committee door-to-door canvass, polling suggests that the bill is unpopular. In polls this week from CNN and NBC News, 33 percent and 24 percent, respectively, of voters said they supported the bill.

The Republican response: just wait. Millions of people now telling pollsters that the tax cut won’t help them will, in fact, pay lower taxes.

“When people see their paychecks get bigger in February because withholding tables have adjusted to reflect their tax cuts, when businesses are keeping more of what they earn, when they can write off their expenses and investments in their businesses and hire more people, that is going to change its popularity, I am convinced,” House Speaker Paul D. Ryan (R-Wis.) said Wednesday on “CBS This Morning.”

Democrats are less convinced. One reason, frequently cited, is the payroll tax cut of 2009, which handed the average worker nearly $700 back during the Great Recession — with zero political benefit. In an early 2010 CBS News/New York Times poll, 12 percent of voters realized that President Barack Obama had cut their taxes; twice as many thought, incorrectly, that Obama had raised them. (The cut was part of the 2009 stimulus package.)

These tax cuts, arriving during a period of economic growth, might be more noticeable. But Democrats are raring to point out the difference between what Republicans ran on and what they passed. The most memorable visual symbol of the tax-cut push, a hypothetical postcard to demonstrate the simplicity of the GOP’s tax plan, disappeared as Republicans put together a compromise that expanded the number of tax brackets and left many loopholes intact.

“Did they wave around that postcard today, or did they forget that?” joked Rep. Bill Pascrell Jr. (D-N.J.), a member of the House Ways and Means Committee.

Democrats, Pascrell pointed out, had endorsed previous tax plans with fewer (and lower) rates than the product that made it through the House and Senate. “Obama had it at 28 percent! Charlie Rangel and I had it at 25 percent 10 years ago. This bill is going to be the gift that keeps on giving.”

Republicans’ own polling has found the same weaknesses as public polling. In a study shared with The Washington Post’s James Hohmann, the GOP-leaning firm Public Opinion Strategies came up with four strong messages in favor of reform. Two of them — “removes and eliminates many loopholes so special interests start paying their fair share” and “simplifies and reduces taxes for most Americans by doubling the standard deduction” — are already being used by Democrats to attack the bill.

The idea of the bill as a corporate giveaway was key to Democrats’ final pre-vote messaging, including a moment in the Senate debate when Sen. Sherrod Brown (D-Ohio) flung open  one of the Senate’s doors and pointed to McConnell’s office to dramatize the influence of lobbyists.

It will also be a key part of a 2018 campaign by the #NotOnePenny coalition, formed by progressives to oppose the tax cut. Next year, the coalition will up its media buy from $5 million to $10 million, hold 100 days of anti-tax cut events, and rally on April 15 in Washington “against tax policy that further rigs the economy in favor of the wealthy.”

On Wednesday, Democrat after Democrat previewed those themes. Rep. Jacky Rosen (D-Nev.), a candidate for Senate next year, released a web video in which Medicaid and Social Security recipients worried that Republicans had put their care at risk. Jenny Wilson, a Salt Lake City council member running for Senate in Utah, dramatized the effects of the bill with “lumps of coal and well-stuffed stockings” to dramatize the bill’s differing benefits for the poor and the very wealthy.

“Washington chose to cut taxes for millionaires and corporations and pay for it by cutting Medicare and kicking people off their health insurance,” said Brown, one of 10 Democrats seeking reelection next year in a Trump-won state, after the vote. “It won’t stop there — congressional Republicans are already planning to steal the money Ohioans have paid into Medicare and Social Security to pay for the hole they are blowing in the deficit.”

Democrats are also watching an X-factor in tax messaging: the president himself. Between them, the American Action Network and the 45Committee, political groups aligned with Ryan and Trump respectively, spent $40 million on ads selling the tax cuts as life rafts for middle-class families.

The president often did the same, promising a “middle class miracle” — and promising that he would not benefit from tax changes, a statement that seems to stand at odds with the bill’s benefit to pass-throughs like the Trump Organization. But at other times, the president has described the bill’s main purpose as help for corporations.

“Our plan also lowers the tax on American business from 35 percent all the way down to 21 percent,” Trump said Wednesday morning at a televised Cabinet meeting. “That’s probably the biggest factor in this plan.”

Within hours, Senate Minority Leader Charles E. Schumer (D-N.Y.) was reading the quote back to reporters.

“It’s no surprise that Donald Trump admitted this morning was the biggest factor in their plan,” Schumer said.