U.S. Senate Democrats on Wednesday blocked a bill that would crack down on Syrian and Iraqi refugees coming to the country as the debate turned into a referendum on Republican presidential candidate Donald Trump and his policies.

The democratic senators sought to force election-year votes as Trump — who holds a commanding lead in national polls for the Republican presidential nomination — has called for barring Muslims from coming to the United States. Republicans have wanted similar votes on politically fraught amendments.

The Senate fell short of the 60 votes needed in the 100-member chamber to move ahead. The vote was 55-43.

The House legislation would require new FBI background checks and individual sign-offs from three high-ranking federal officials before any refugee from Syria or Iraq could come to the United States. The American Security Against Foreign Enemies Act cleared the House in November in the aftermath of the Paris attacks. It received 289 votes, a veto-proof margin that included 47 Democrats — despite President Barack Obama's opposition.

'Another step in the absolute wrong direction'

"This bill is just another step in the absolute wrong direction, the direction of Donald Trump," Senator Harry Reid, the Democratic minority leader, told reporters before the vote. "The Democrats are committed to opposing the hateful views of Trump and his Republican enablers."

But Senate Republicans who backed the House bill said it is difficult to effectively vet immigrants from war-torn countries like Syria and Iraq, where record keeping is poor — or may not exist at all. They also said senior U.S. intelligence and law enforcement officials have expressed concern that the Islamic State may try to exploit the refugee screening program.

"So it is any wonder that the citizens we represent are concerned?" Republican Senate majority leader Mitch McConnell said. "No wonder dozens of Democrats joined with Republicans to pass this balanced bill with a veto-proof majority over in the House."

Presidential candidates vote

Three of the Republican presidential candidates — Ted Cruz of Texas, Marco Rubio of Florida and Rand Paul of Kentucky — left the campaign to return to Washington to vote for moving ahead on the measure.

Democratic presidential candidate Bernie Sanders of Vermont missed the vote as did Republican Senator Lindsey Graham of South Carolina, who was campaigning for Jeb Bush in New Hampshire.

Two Democrats from Republican-leaning states — Heidi Heitkamp of North Dakota and Joe Manchin of West Virginia — voted with Republicans to move ahead on the legislation.

May put Democrats in difficult position

For Democrats facing tight 2016 elections, opposing the bill may put them in the difficult position of rejecting what many consider to be a reasonable anti-terror measure in the wake of a terrible tragedy. Those concerns surfaced ahead of the House vote in November when White House aides went to the Capitol to win over Democrats in a private meeting. 

Representative Sean Patrick Maloney, a Democrat, told them, in a forceful exchange, that voting no could hurt Democrats at the polls, according to aides in attendance.

Reid said Democrats also wanted to propose an increase in anti-terrorism money for local police forces and airport security and banning the sale of guns and explosives to people on federal terrorism watch lists.