WASHINGTON (MarketWatch) — As President Barack Obama and members of the Senate head back to Washington, 60 is the most important number.

It will take 60 votes for any scaled-back deal on the fiscal cliff to clear a filibuster in the Senate.

“All eyes are on the Senate in the next 48 hours,” said Chris Krueger, senior policy analyst at Guggenheim’s Washington Research Group.

There are just five days remaining until automatic spending cuts and the expiration of Bush tax cuts kick in. On Wednesday, U.S. stocks gave up early gains to finish lower, with the S&P 500 Index SPX, +0.82% down 0.5% at the close.

Obama and Senate Majority Leader Harry Reid are expected to put together a scaled-down deal to blunt the fiscal drag scheduled after Dec. 31. Read: President presses for smaller fiscal-cliff deal.

Krueger said the “small-ball” bill would extend tax cuts for those making $250,000 or less. This would result in an increase in tax rates for wealthy Americans. The measure is also expected to extend unemployment insurance and set the top tax rates on capital gains and dividends.

The biggest uncertainty remains how Senate Republican Leader Mitch McConnell and his Republican colleagues would react to such a measure. At least seven Republicans would have to join Democrats to pass any fiscal-cliff plan through the Senate.

McConnell is up for re-election in 2014. Democrats question whether he will be able to work with Obama and Reid to pass a fiscal-cliff deal. Senate Republicans are not keen to vote to raise taxes if the plan is not going to win approval in the House of Representatives.

The House remains in disarray after House Speaker John Boehner failed late last week to pass his version of a fiscal-cliff deal. See: House Republicans cancel “Plan B” tax vote.

In a statement late Wednesday, Boehner said the House would consider whatever legislation the Senate passed. “The House will take … action on whatever the Senate can pass, but the Senate must act first,” he added.

Fiscal cliff: So close and yet so far

Obama will fly out from Hawaii late Wednesday back to Washington. The Senate has votes scheduled for Thursday. The House remains in recess; its members can return within 48 hours if leaders call them back.

“It remains very unclear if there are the votes to pass anything in the next five days,” Krueger wrote to clients.

Robert Reich, a former member of President Bill Clinton’s cabinet, said the White House’s hope of getting the Senate to pass a smaller fiscal-cliff deal that raises taxes on the wealthy won’t work because the measure can’t get through the House.

In a column for Business Insider, Reich said Obama should go over the cliff and force the Republicans’ hand. Early next year, Democrats could introduce legislation that restores the tax break for the middle class retroactive to Jan. 1. Republicans would then feel compelled to go along, Reich added.

Guggenheim’s Krueger said “there is a 70% chance that we go over some form of the cliff at the end of the year. There is a lot of maneuvering between all sides, to be sure; they still have a chair when the music stops on Jan. 1 and the blame game could reach full swing by the weekend.”