Donald Trump was in full salesman mode on Wednesday evening as he tried to build momentum behind his plan to overhaul America's tax code and revive his stalled legislative agenda.

Hours after the White House and congressional Republicans released a framework for sweeping changes to the tax system, the President told hundreds of supporters at the Indiana State Fairgrounds the plan was "a once-in-a-generation opportunity".

"This is a revolutionary change and the biggest winners will be the everyday American workers as jobs start pouring into our country, as companies start competing for American labour, and as wages start going up at levels that you haven't seen in many years," he said.

Passing the tax plan has become critical for a President desperate for a win.

Mr Trump has faced repeated, embarrassing setbacks, including Republicans' failure to repeal and replace former President Barack Obama's health care law. Short of votes, Senate Republicans announced on Tuesday that they would not vote on the latest health care proposal.

The tax plan seeks to slash the corporate rate from 35 per cent to 20 per cent and create three individual tax brackets with rates of 12 per cent, 25 per cent and 35 per cent, with a recommended surcharge on the very wealthy. Mr Trump also wants to simplify the tax code to allow the majority of Americans to file on a single sheet of paper.

Calling his plan a "giant win for the American people", he derided the current tax system as a "relic" and a "colossal barrier" standing in the way of the nation's economic comeback.

"We're going to remove that barrier to create the tax system that our people finally, finally, finally want and deserve," he said.

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The sales pitch also offered more evidence that Mr Trump would — unlike with health care — make an attempt to cultivate a small group of moderate Democrats who might be willing to cross the aisle and back his plan. Congress has not approved significant changes to the tax system since 1986, at the height of President Ronald Reagan's popularity and after extensive hearings and deliberations.

Joining Mr Trump aboard Air Force One was Indiana Senator Joe Donnelly, who is among the most endangered Senate Democrats facing re-election in 2018. Mr Donnelly has expressed openness to the tax overhaul as part of an effort to penalise companies for offshoring.

But instead of buttering him up, Mr Trump issued a warning shot to Mr Donnelly from the stage: Support my plan or I'll campaign against you next year.

"If Senator Donnelly doesn't approve it — because you know he's on the other side — we will come here and we will campaign against him like you wouldn't believe," Mr Trump said. Still, he predicted that "numerous Democrats" would choose to back the plan because "it's the right thing to do".

"Democrats and Republicans in Congress should come together, finally, to deliver this giant win for the American people and begin middle class miracle — it's called a middle class miracle — once again," he said.

The President has also made overtures to Democratic senators like Claire McCaskill of Missouri and Heidi Heitkamp of North Dakota in recent weeks. All three face re-election in 2018.

Mr Trump urged those gathered to call their congress members to pressure them to pass the plan.

"You just want massive tax cuts, that's what you want," he told his audience as he took the stage to cheers. "That's the only reason you're going so wild."