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ABC News' Arlette Saenz and Jeff Zeleny report:

WASHINGTON - Business and labor leaders reached an agreement late Friday night on a guest worker program for low-skilled immigrants, a senior Democratic Senate aide tells ABC News, marking a key moment in the immigration debate.

The agreement involves the guest worker provision for low-skilled immigrants, which is one of the most critical pieces of the legislation. The accord between the Chamber of Commerce and the A.F.L.-C.I.O marks the first time the dueling sides have come together around the size and scope of the guest worker program.

"This issue has always been the dealbreaker on immigration reform, but not this time," Sen. Chuck Schumer said in a statement Saturday night.

Schumer, D-N.Y., one of the leading Senate negotiators on the agreement, briefed White House chief of staff Denis McDonough on the deal this afternoon, ABC News has learned.

"We are very close, closer than we've ever been," he said. "We are very optimistic, but there are a few issues remaining."

The agreement between business and labor was reached shortly after 9 p.m. Friday, when Schumer convened a conference call with the top labor leader, Richard Trumka, and the chamber head, Tom Donohue. The three agreed to have dinner soon to commemorate the accord, one official tells ABC news.

The sticking point in earlier negotiations centered on determining pay levels for future immigrant workers coming to the country on new visas. The unions wanted employers to pay an average wage for occupations rather than assigning salary by skill levels, while the business side called for paying low skilled workers at the lowest rate.

The guest worker deal reached by business and labor will now be presented to the bi-partisan Gang of Eight senators involved in the immigration talks. The senators are also working on fine tuning the path to citizenship and border security components of the plan.

"Senate negotiators are making good progress on immigration reform, but we're not done yet," Alex Conant, a spokesman for Sen. Marco Rubio, R-Fla., tweeted today.

While the group of senators works on the details of the immigration plan, President Obama said this week that he is optimistic the group will produce a bill in April.

"I'm actually optimistic that when they get back they will introduce a bill," Obama said during an interview with Univision earlier this week. "My sense is that they have come close and my expectation is that we'll actually see a bill on the floor of the Senate next month."