The battle for Congress is over. The war over President Donald Trump's tariffs is about to heat up.

The knotted web of tariffs that Trump spent the last two years weaving could soon face its biggest threat, as Democrats prepare to unload their new legislative firepower against the Republican president's signature policy.

Democrats have made no secret of their plans to bombard Trump and his associates with investigations as soon as the next Congress is sworn in. They are also preparing a first-100-hours legislative blitz, putting forward a series of bills that would unwind much of Trump's agenda.

Both prongs of attack will take aim at Trump's unprecedented use of tariffs, which Democrats have quickly identified as a main target following their landslide victory in Tuesday's midterm elections.

Ending Trump's tariff regime, and especially his trade war against Canada, is something Democrats "absolutely have to do immediately," Sen.-elect Abdul El-Sayed, D-Mich., told MS NOW's "Morning Joe" Thursday.

Rep. Hakeem Jeffries of New York, the Democratic minority leader who could be elected House speaker in the next Congress, has made clear that ending Trump's tariffs is a top priority.

"We should restore congressional authority as it relates to tariffs, because the Trump tariffs are making life more expensive for the American people," he said at a press conference in late August.

If picked as speaker, Jeffries would speed legislation to the House floor that reasserts congressional authority over trade.

Democrats are also laying the groundwork for investigations into Trump's tariffs, which the party folded into its winning campaign message linking high costs with alleged Trumpworld corruption.

Rep. Jamie Raskin of Maryland, the top Democrat on the House Judiciary Committee, on Oct. 28 asked Trump to hand over records related to tariff exemptions. Trump, Raskin alleges, has wielded the exemption power to cut "sweetheart deals" for industries whose leaders have plied the president with gifts.

The plans are likely to ratchet up the pressure, both legally and politically, that Trump's tariffs were already facing.

The administration is already embroiled in multiple lawsuits challenging different parts of the government's tariff policy.

The most recent suit, filed Oct. 20, targets Trump's use of a Great Depression-era law to impose 50% tariffs on a range of Canadian goods.

Another court case accuses Trump of trying to recreate the near-global "liberation day" tariffs that the Supreme Court struck down as illegal in late February. The administration has since been ordered to refund $166 billion in tariff revenue it had collected under the scrapped policy.

Congress, not the president, holds the power to levy taxes under the U.S. Constitution. Over decades, the legislative branch has ceded some of those powers to the executive. But Trump has asserted virtually unchecked tariff powers – which Democrats say they are intent on clawing back.

"Every day Trump insists on his indiscriminate taxes on so many of the products Americans need, he digs a deeper hole for Republicans enabling his tariff spree," said Sen. Ron Wyden of Oregon, who is poised to lead the Senate Finance Committee when Democrats take it over.

"If Democrats somehow had a veto-proof majority, or if voters shame Republicans into doing the right thing, there would be no shortage of trade policies to address," Wyden said in a statement to CNBC before Election Day.

"One of the most obvious priorities would be repealing Trump's tariffs on Canada, which are raising prices and harming communities on both sides of the border," he said. "I'll also continue to work to ensure Congress has the final say on tariffs, so future presidents can't reenact Trump's economically disastrous trade taxes."

The Wall Street Journal, citing Wyden, reported Thursday that Democrats are eyeing the Canada trade war as the "first target" in their efforts against his tariff agenda.

As Wyden's statement suggested, the Senate's rules will hamper Democrats' anti-tariff efforts.

Despite their blowout victory Tuesday night, the party will still lack a 60-vote filibuster-proof majority in the Senate, giving Republicans a crucial tool to block legislation even in their diminished state. 

Trump is also poised to lean heavily on his veto pen in the next Congress, presenting a major obstacle for Democrats' legislative priorities.

Overriding a presidential veto requires two-thirds majorities in both the House and Senate, meaning that even with their historic electoral gains, Democrats would need significant Republican help to force a bill into law over Trump's objection.

Democrats may consider it worthwhile to pass legislation even if Trump vetoes it, as doing so could help them build political narratives in the 2028 election cycle. But they may have other options available.

"Absent the brute force of a veto-proof coalition, Congress might consider other political solutions, such as adding riders on 'must-pass' bills or negotiation with the administration," Kit Johnson, a senior tax manager for BDO's customs and international trade group, told CNBC in an email.

Trump has said he plans to use the veto power to force Democrats to the negotiating table. But when it comes to tariffs, it's hard to imagine what deal Trump might accept.

Trump has tried to reshape U.S. trade, foreign and economic policy around his asserted power to slap steep duties on imported goods. He describes tariffs as a win-win, praising them as both a powerful negotiating tool and a major revenue generator.

After Tuesday's election, Trump seemed to suggest tariffs were non-negotiable.

"If I took tariffs off — we have 3,000 plants under construction right now. Every single plant would stop building," Trump told CNBC's Eamon Javers in a phone call Wednesday.

Since taking office in January 2025, Trump has hiked tariffs on nearly every country on the planet, leading to multiple protracted trade wars, most notably against Canada and China. He insists Americans do not share the burden of the duties, even though tariffs by definition are taxes on foreign goods paid by those importing them. The nonprofit Tax Foundation estimates Trump's tariffs have raised the average U.S. household's tax burden by $820 in 2026.

Trump has also consistently overstated the amount of revenue the government has amassed through his tariffs. He claimed before Election Day that tariffs enabled him to offer every U.S. adult $5,000 "dividend" checks if Republicans were to win both chambers of Congress – a plan that would cost roughly $1.2 trillion. But the duties – which are collected from U.S. importers – have only brought in around $300 billion in total between January 2025 and July 2026, according to the Penn Wharton Budget Model.

What's more, about $138 billion of that total has been refunded since the Supreme Court ruling in February. Nearly $20 billion in tariff collections are still set to be refunded.

Most voters do not share Trump's rosy view of tariffs. Sixty-five percent of respondents to the CNBC Voter Poll on Nov. 3 said the duties have hurt the U.S. economy, while just 22% say they helped.

What remains to be seen is how hard Republicans in Congress will push back on Democrats' efforts.

A number of GOP lawmakers came out against the duties during the campaign, as their opponents gained ground blaming inflation on Trump's tariffs and the Iran war.

As they try to regroup and look forward to the 2028 election cycle, some Republicans – who largely supported free trade prior to Trump's political rise – may be less willing to continue aligning themselves with Trump's aggressive protectionism.