The letter comes amid a concerted push by Republicans to mobilize support for their plan to pursue a large tax overhaul package this year, with Trump pressing Cabinet members about the goals in the morning and three top advisers mapping out the road ahead later in the day. They cited Treasury Secretary Steven Mnuchin's statement during his confirmation hearing that there would be "no absolute tax cut for the upper class", although he has since backed away from that statement.
But so far Democrats have largely been excluded from the process.
Democrats were also critical of the possibility of Republicans using the legislative process known as reconciliation to pass tax reform. It suggested that the wealthiest individuals had already seen "outsized benefits from recent economic gains while working-class wages have remained stagnant".
Fresh off their failure to pass a health care bill, Republicans have sought support for reconciliation to more quickly pass tax legislation in the fall.
"We will need to use reconciliation because we have been informed by the majority of the Democrats in a letter I just received today that most of the principles that would get the country growing again, they're not interested in addressing", McConnell said, leaving the option for Democrats to support a Republican-led tax plan.
Currently, the tax reform effort is still in its infant stages.
After the Trump administration and the top Republican tax negotiators in Congress released a set of common tax principles last week, Democrats warned that they were going down the wrong path.
When a reporter asked McConnell if Republicans could agree to the Democrats' demand not to add to deficits, he said, "Oh, tell them to grow up".
The conditional offer may not attract immediate response from Republicans.
But how the rich are taxed could be the biggest source of contention.
"We need sustainable, comprehensive tax reform, not a massive tax cut for the wealthy", Mr. Wyden said.
The Senate and House have 12 joint working days before September 29, when the Treasury Department says it would no longer be able to pay all of the government's bills unless Congress acts.
Attention has been focused on the White House, where Trump has repeatedly threatened to cut off federal disbursements insurers use to lower out-of-pocket costs for low- and moderate income consumers buying individual policies. And so any hope that checks and balances would work to constrain Trump's worst impulses hinged, in part, on the willingness of Republicans in Congress to act in defense of values higher than short-term political advantage, or at least to interpret their short-term political interest as requiring them to counter Trump.
- As Republican lawmakers promote an overhaul of the tax code, Senate Democrats are offering conditions of their own.