"Reports that the bipartisan group of eight senators have agreed on a legislative proposal are premature," said Rubio, a Florida Republican who is among the lawmakers working on legislation.
Rubio, a Cuban-American who is weighing a presidential bid in 2016, is a leading figure inside his party. Lawmakers will be closely watching any deal for his approval and his skepticism about the process did little to encourage optimism.
Rubio, who is the group's emissary to conservatives, called the agreement "a starting point" but said 92 senators from 43 states haven't yet been involved in the process.
The detente between the nation's leading labor federation and the powerful business lobbying group still needs senators' approval, including a nod from Sen. John McCain, the Arizona Republican whose previous efforts came up short.
"I think we're on track. But as Sen. Rubio correctly says, we have said we will not come to final agreement till we look at all of the legislative language and he's correctly pointing out that that language hasn't been fully drafted," Schumer said.
Sen. Jeff Flake, R-Ariz., also noted the significance of the truce between labor and business but added that this wasn't yet complete.
"That doesn't mean we've crossed every 'i' or dotted every 't,' or vice versa," said Flake, who is among the eight lawmakers working on the deal.