Senate Majority Leader Harry Reid (D-Nev) scheduled a decisive vote Monday on a much-anticipated economic stimulus package, a move that could force the top presidential candidates to return to Washington in the final hours before Tuesday’s crucial primaries.

The cloture vote to call up the stimulus package is set for 5:30 Monday evening. That vote would occur the night before polls open in more than 20 states holding presidential primaries on Tuesday.

The schedule could put three of the four leading presidential contenders in both parties — all sitting senators — in a quandary if their votes would make a difference in Democrats securing the 60 votes necessary under Senate rules to bring up the stimulus package approved by the Finance Committee earlier this week. However, if Reid isn't close to having enough votes, the timing may be a sign that he's ready to move on and accept a rival package worked out in a deal between House leadership and the White House. 

For the Democratic presidential candidates, coming back to help their party pass the legislation would mean leaving their campaigns at a vital moment.

Reid, who was scrambling Thursday to get the 60 votes needed to advance his party’s measure, said he “absolutely” expects Sens. Barack Obama (D-Ill.) and Hillary Rodham Clinton (D-N.Y.) will return to Capitol Hill Monday even though they are embroiled in a ferocious battle for their party’s presidential nomination.

“They’ve never missed a [big] vote,” Reid said. “Whenever I’ve asked them to come, they’ve come.”

Lawmakers expect the presidential race increasingly to have an impact on the Senate’s schedule, with the growing possibility that each party’s nominee may end up being a sitting senator.

Monday's vote presents risks for McCain, too. By breaking ranks, he could be criticized by rivals for supporting the plan crafted by Senate Democrats. But by voting against the Senate economic package, he could be criticized for opposing provisions for the elderly and veterans.

The Finance bill includes  tax rebates of $500 for seniors and disabled veterans and an extra 13 weeks of unemployment insurance  that were not included in a House bill forged in a delicate deal between House leaders and President Bush last week. The Senate bill also sets income ceilings of $150,000 for individuals and $300,000 for couples at which tax rebates would phase out, double the ceilings in the House bill.

If Reid's move fails to win the 60 votes needed to break off a GOP-led filibuster, Democrats will try to force a vote on a new package that includes a number of measures favored by their conference, including low-income heating assistance.

Republicans aren’t planning on offering amendments. Instead, GOP leaders will call on Reid to quickly approve the House plan so Bush can sign a stimulus plan into law. The GOP derided the measure as a “Christmas tree,” but Democrats are daring Republicans to vote against a plan with provisions to help the elderly and veterans.