PITTSBURGH — With his jobs bill facing likely defeat in the Senate on Tuesday, President Obama called on his supporters to demand an explanation from any senator who votes against the $447 billion proposal.

“This is the moment of truth for the U.S. Senate,” Obama said during an appearance here at a training facility for electrical workers. “Today’s the day when every American will find out exactly where their senator stands on this jobs bill. ... Any senator who votes no will have to look you in the eye and tell you what exactly they are opposed to.”

Obama has been barnstorming the country behind his American Jobs Act, demanding that Congress vote on the plan, which includes a mix of tax breaks for small business owners and infrastructure investments. Senate Democrats have pledged to hold a vote on the bill Tuesday.

But Republicans, and some Democrats, have opposed portions of the proposal, and the bill is unlikely to win the necessary 60 votes in the Senate to avoid a Republican filibuster. Democratic leaders were scrambling to try to win at least 51 votes from their own caucus.

The White House has tried to use the bill as a political wedge to force Republicans to negotiate or risk being painted as unwilling to tackle the economic crisis. On Tuesday, Obama, speaking before a crowd of several hundred at a training facility of the International Brotherhood of Electrical Workers, rallied his supporters to ensure that his opponents pay a political price.

“I need your help. Your senators are voting today on this jobs bill. This is gut-check time,” Obama said. “They will have a hard time explaining why they will vote no on this bill other than that I proposed it.”

But even as Obama spoke, Republicans continued to voice strong opposition to the spending measures in the package, as well as proposed increases in tax rates on higher-income Americans that would pay for it.

“The President’s been calling for this vote for weeks; and in my view, we can’t have it soon enough,” Senate Minority Leader Mitch McConnell (R-Ky.) said in a statement. “By proposing a second stimulus, Democrats are showing the American people that they have no new ideas for dealing with our jobs crisis. Today’s vote is conclusive proof that Democrats’ sole proposal is to keep doing what hasn’t worked — along with a massive tax hike that we know won’t create jobs.”

Before making his remarks, Obama toured the training facility and presided over a meeting of his Council on Jobs and Competitiveness. The president said that if his jobs bill is defeated in the Senate, he would consider breaking up the proposals in the legislation into individual bills and move them independently.

Republicans have said they would consider some of the ideas in the jobs plan that they have supported in the past, such as the tax breaks for small business owners.

Obama invoked the late Apple co-founder Steve Jobs, who died last week, at the beginning of his remarks, citing him as the kind of entrepreneur that “America is all about.”

“The spirit of those entrepreneurs and innovators are how we became the world’s leading economic power,” Obama said. “But right now, our economy needs a jolt. Right now.”