Arguing with Red about the constitutionality of taxation (or anything really) seems an enormous waste of time.
What the Dems need to do is explain WHY they want to tax the wealthy. Not just for the increased revenue, but what they will do with it. But also the pernicious affects of inequality, which contributes to the gutting of the middle class and the polarization of politics. Expose the GOP plutocratic policies and appeals to fears and prejudice (to get the underclass on board with plutocratic policies). Point out how tax cuts always lead to more debt and benefit the few.
I think the first proposal should be to rescind Trump's tax cut for the wealthy. Without that Obama tiptoeing where the Bush cuts were extended a few years before being phased out.
2nd, corporations should have to pay a minimum profit tax of 15%.
No more zeroing out taxes for years and decades. Scandalous that billion dollar profits are often not taxed at all through the wonders of accounting and tax loopholes. I'd use Trump's debate line about No company should pay more than 15% tax and flip it so that no corp pays less than that. We give corporations special privileges and liabilities in order to accumulate wealth. And when they do, a portion of that should be kicked back.
Talk about the declining tax income from companies and the increased burden on individuals.
Dems need to talk about history. How income tax rates often were way higher than today. that companies used to pay a much greater share of federal tax revenue. That the 60's boom featured rates as high as 90%. The Clinton Surplus came after a tax increase. Dems need to sustain narratives and make their case.
Partly you tax billionaires and corporations becaue "that's where the money is." But also out of fairness. And also to improve the public good, benefit the people and improve the country.
I'd also be announcing a giant infrastructure building project (jobs!, improvements!) with some of the new tax money. Thew ay forward is pretty clear, but step by step and explaining is also part of the way it gets done.
The Dems also need to consistently and persistently make the case for why they need control of the Senate. And why they need to have unified gov't control.