Senate Majority Leader Harry Reid, D-Nev., said Thursday Toomey and Portman have failed to explain how they can increase tax payments while cutting taxes.
He called their proposal "phoney."
Sen. Patty Murray, D-Wash., co-chair of the deficit reduction supercommittee, had a similar response.
But Sen. Richard Durbin of Illinois, the Democratic whip, said he is encouraged by any discussion from the other side of the aisle of raising revenues. He said he does not support the specifics of the Toomey-Portman plan.
The two Republicans met Wednesday with members of the Gang of Eight, the bipartisan group trying to find common ground on the budget, sources told The Hill.