WASHINGTON (CNN) -- A bipartisan group of senators and administration officials, racing to meet a Wednesday deadline to agree to comprehensive immigration reform bill, will meet Tuesday afternoon in what Sen. Trent Lott, R-Mississippi, called a "make or break" session to see if they can reach a deal.

Optimism expressed by negotiators Monday was replaced by words of caution Tuesday that much work remains.

"I'm increasingly concerned we will not" reach a deal," said Sen. John Cornyn, R- Texas, who is heavily involved in the talks.

"We still have a ways to go," another leader in the talks, Sen. Edward Kennedy. D- Massachusetts, told reporters. "We're still talking."

A GOP leadership aide said, "There are dozens of issue still to be resolved."

Majority Leader Harry Reid, D-Nevada, has scheduled a key test vote for Wednesday afternoon. Republicans said they hoped Reid would push back the vote to give talks more time.

"I'm still hopeful," Lott said. "I think we should go forward but it may take a little while."

Negotiators declined to discuss the details of their differences but several said they can't get a deal on the overall bill until each of the underlying issues is resolved.