WASHINGTON (Reuters) - The tax bill is President Donald Trump’s biggest legislative victory this year, but Democratic strategists are already planning how to turn it into his biggest liability.

FILE PHOTO: Political and climate activist Tom Steyer speaks while taking part in a protest against U.S. President Donald Trump and Republican congressman Darrell Issa (R-Vista) outside Issa's office in Vista, California, U.S., October 31, 2017. REUTERS/Mike Blake

The emotional trigger they think will work on voters at next year’s midterm elections: arguing the bill is profoundly unfair by giving the lion’s share of benefits to corporations and the rich.

“Be careful what you wish for,” Tom Steyer, a Democratic billionaire, said of Republican leaders in an interview with Reuters on Wednesday. “They wanted this, they should have never have wanted it. Now that they’ve got it, they’re going to wish they didn’t.”

Steyer said he plans to use his money - through his political group NextGen America - to attack the Republican tax overhaul, using social media and online advertising aimed at young voters, a strategy that recently helped elect Democratic candidate Doug Jones in a Senate race in Alabama and Ralph Northam as governor of Virginia.

In November 2018, elections will be held for all 435 seats in the U.S. House of Representatives and for 34 seats in the 100-member Senate.

Pollsters believe the midterms could be a rare “wave election,” when one party seizes back control of Congress. It happened for Republicans in 2010 and 1994.

If Democrats are able to pick up two more Senate seats, they will take control of the chamber. And after the wins in Alabama and Virginia, they are even hopeful they could tap into anger over tax cuts for the rich to win control of the House.

Only about 29 percent of voters approve of the tax bill, according to a Reuters/Ipsos poll conducted in mid December. Slightly more than 52 percent oppose it. In no region of the country did even half of the people polled say they support it.

The Democratic Senate Campaign Committee is already running anti-tax plan ads in several key states including Nevada, Arizona, Indiana, Ohio, Pennsylvania and Wisconsin.

The ads are only five-second spots running before YouTube videos. The viewer, unable to skip over the ads, will hear: “The Republican tax scheme gives huge breaks to corporations but raises taxes on middle class families.”

The tax bill cuts the corporate tax rate to 21 percent and raises the threshold at which an inheritance is taxed. It also cuts the tax rate for top earners.

While it also cuts the tax rate for most other income groups and doubles the size of the standard deduction, the elimination of other popular deductions could result in some taxpayers seeing a tax increase instead of a cut. It has been popular with the Republican base, Trump voters and the party’s donors. The stock market has surged in anticipation of the cuts.

Democrats plan to focus on provisions like benefits to commercial real estate owners, arguing that the tax bill could produce millions of dollars of more cash for Trump and his family members. The White House asserts Trump will not personally benefit – but it admits that his business might.

The Super PAC American Bridge, which supports Democratic candidates, is running digital ads in states with Senate races targeting women, swing voters and Republicans in suburban areas, said Joshua Karp, the group’s communication director for Senate races.

“I think there is a couple of things that get almost all Americans riled up about this bill, it is fundamentally unfair and breaking the promise the Republicans made to the American people,” Karp said.

Republicans say Democrats are indulging in wishful thinking. Polls say 50 percent of voters think their taxes will go up, but tax analysts say that 80 percent will pay less tax.

Republicans are counting on voters, when they see more money in their paychecks, to dismiss Democratic rhetoric, and perhaps even throw their support behind Trump.

And if there are any unpleasant surprises in store for taxpayers, they will not be filing their first tax returns under the new law until early 2019, months after the midterm elections.

Republican strategist Alex Conant, a veteran of congressional and presidential campaigns, said the unpopularity of the tax cuts in opinion polls can be directly attributed to Trump’s low approval levels.

“We’re losing elections because Donald Trump is incredibly unpopular and makes a lot of independents and soft Republicans uncomfortable ... Trump’s numbers are dragging down tax reform’s numbers,” Conant said, although he also played down the electoral risks of the tax overhaul.

“I would be very surprised if people are marching in the streets months from now because we cut their taxes.”

Republicans have some other advantages, like the growing economy, that means an attack on the tax overhaul could backfire.

“Democrats will have a very hard time taking this road if the economy is still going gang busters 11 months from now,” Republican strategist Joe Bretell said.

But he acknowledges that the Democrats are on the right track.

“The emotional trigger point is in fairness,” he said. “The party that can best explain or brand this bill and explain why it’s fair or unfair to their base or the other side, wins.”