(Politico) — President Barack Obama’s jobs plan is at risk of getting less than 51 votes Tuesday evening in the Senate as a handful of politically vulnerable moderates hold out on the president’s signature economic proposal.

Adding to the uncertainty, Sen. Jeanne Shaheen (D-N.H.) who supports the proposal, may be a no-show due to a scheduling conflict, potentially leaving Democrats short of the symbolic simple majority on the jobs bill.

Discussions are still fluid between wavering Democrats and Senate Majority Leader Harry Reid and his leadership team — so it remains to be seen how the final vote shakes out. And even if Obama won support from his entire 53-member Democratic Caucus, he would have failed to reach the 60 votes needed to break a GOP-led filibuster.

But falling short of a simple majority in the Democratic-led Senate would be an embarrassment for the president, and provide even more fodder for Republicans who have objected to the White House’s repeated calls to pass his jobs plan “now.”

Sen. Ben Nelson (D-Neb.) said last week he’d vote to filibuster the plan, and Sen. Joe Manchin (D-W-Va.) is also expected to vote against it Tuesday evening. Shaheen is scheduled to attend a previously planned event in Boston Tuesday where she’ll be awarded the “New Englander of the Year” by a regional business council. It remains to be seen whether Shaheen will return to Washington Tuesday evening.

Without those three votes, there would only be 50 votes in support of cloture Monday evening because all 47 Republicans are expected to vote against considering the plan. Sen. Jon Tester (D-Mont.) also has said he’d oppose the president’s jobs plan, but has not said how he’d vote on the motion to break the filibuster.