Senate Minority Leader Mitch McConnell (R-Ky.) and Vice President Joe Biden engaged in furious overnight negotiations to avert the fiscal cliff and made major progress toward a year-end tax deal, giving sudden hope to high-stakes talks that had been on the brink of collapse, according to sources familiar with the discussion.

McConnell and Biden, who served in the Senate together for 23 years, only started talking Sunday, after negotiations between Senate Majority Leader Harry Reid (D-Nev.) and McConnell sputtered.