The White House refused to admit defeat despite signs a bid to expand background checks for gun owners will fail later on Wednesday in the US Senate.

President Barack Obama's spokesman Jay Carney also lashed out angrily at complaints by Republican Senator Rand Paul that the White House was using families who lost kids in the Newtown school massacre as "props."

A clutch of votes was expected in the Senate later on Wednesday, including on the push to expand background checks for gun purchases that has become Obama's main focus in reforming gun laws after last year's school killings.

But there are increasing signs that the effort, opposed by most Republicans, the powerful gun lobby and some Democrats in conservative or rural states will not get the 60 votes needed for passage.

But Carney insisted that the drive for enhanced background checks was not yet dead.

"This isn't over. I'm not going to give a postmortem," said Carney.

"There is no question that the path to 60 in this case is difficult, but it is not unachievable.

"If you are opposed to this legislation, you should explain why you are against something that 90 percent of the people are for, that vast majorities of the people in your state support."

White House sources said the president had been calling wavering senators to push for support for the background checks bill in the hours before the vote and Carney said everyone in the White House from Obama on down was involved.

The president's spokesman also flashed genuine anger when he was asked about a reported comment from Paul that Obama had used Newtown families who have been lobbying for reforms on Capitol Hill, as props.

"I don't know if Senator Paul met with the Newtown families. The Newtown families aren't here for the president.

"They're here because their children were murdered. They're here asking for the Senate to do something that is common sense."