Updated

The Senate began debate Monday on a Republican effort to overturn new labor regulations that make it easier and quicker for unions to hold workplace elections. The White House immediately threatened to veto it.

Even though it can't be filibustered and needs only a simple majority to pass, the rarely invoked resolution of disapproval is given little chance of succeeding when the Senate votes on it Tuesday. The vote, however, forces lawmakers to take sides ahead of the November election on an issue that sharply divides unions and business groups.

The new rules were adopted last year by the National Labor Relations Board and are scheduled to take effect on April 30. They are aimed at reducing the time it takes to hold an election after 30 percent of eligible employees at a workplace sign cards saying they want a union.

Republicans and business groups claim the change allows unions to ambush companies, leaving managers without enough time to respond before workers decide whether or not to seek collective bargaining rights.

"This rule will shift the law significantly in favor of big labor," said Sen. Mike Enzi of Wyoming, top Republican on the Senate Health, Education, Labor and Pensions Committee.

Enzi urged his colleagues to "prevent the small business employers out in America from being ambushed and employees from being misled with insufficient information into union contracts that they cannot get out of."

Unions call the changes a modest fix that would limit corporate stalling tactics, which they say can delay elections for months or years in the most egregious cases. Most union elections currently take place 45 days to 60 days after a union gathers enough signatures to file a petition to hold a formal election. Business groups claim the new rules could shorten that time to as few as 14 days.

"Employers always know what's going on and they have ample opportunity to express their views," said Sen. Tom Harkin of Iowa, Democratic chairman of the Senate panel overseeing labor. He said the new rules "just give workers the ability to say yes or no without having to wait months or years to do so."

The new rules could help unions expand in the private sector, where membership has dwindled to about 6.9 percent of all workers.

Senate Republicans needed only 30 signatures to force a vote on the resolution under the Congressional Review Act. The White House said in a statement Monday that President Barack Obama's senior advisers would recommend he veto the measure if it reaches his desk.

AFL-CIO legislative director Bill Samuel said unions are confident they will win the vote in the Senate. "We really don't think the rule is in jeopardy," he said.

The tactic to nullify regulations has succeeded only once since it was first permitted under the Congressional Review Act of 1996. In 2001, after President George W. Bush took office, Congress repealed ergonomic regulations that had been approved by the Occupational Safety and Health Administration under President Bill Clinton.

The union election measure already has the backing of 45 out of 47 Senate Republicans. Only GOP Sens. Scott Brown of Massachusetts and Lisa Murkowski of Alaska didn't sign on as sponsors. They haven't stated publicly how they will vote.

GOP leaders are hoping to win support from a handful of moderate Democrats, including Sen. Ben Nelson, D-Neb., Claire McCaskill, D-Mo., and Jon Tester, D-Mont., who have also not shared their position publicly. Tester, McCaskill and Brown all face tough re-election challenges in November.