President Barack Obama on Tuesday refused to sign a Republican-backed bill that would have overturned controversial union voting rules, Reuters reported; his move marked the fourth time he vetoed legislation during his more than six years in office.

The rules, adopted last year by the National Labor Relations Board and set to take effect April 14, shorten the period between a union filing a petition to represent workers and an election. The current median of 38 days will be cut to as little as 14 days, the newswire detailed.

Republicans in the House and Senate this month approved a resolution that would have stopped the implementation of the new guidelines; Democrats in both chambers opposed that move.

GOP leaders have dubbed the National Labor Relations Board guidelines "ambush election" rules, saying they would allow unions to place undue pressure on workers and employers, the Hill noted. House Speaker John Boehner on Tuesday slammed Obama's veto and accused the president of having no interest in working on legislation that boosts job growth.

The rules mark "an assault on the rights and privacy protections of American workers," Boehner said in a statement. "With his veto, the president has once again put the interests of his political allies ahead of the small business owners and hardworking Americans who create jobs and build a stronger economy," the Ohio Republican added.

But Obama insisted at a press conference that the guidelines represented modest changes that would help workers who choose to unionize, according to Reuters.

"Unions historically have been at the forefront of establishing things like the 40-hour work week, the weekend, child labor laws, fair benefits and decent wages," the president noted.

Rep. John Kline, meanwhile, challenged the president on his assumption that the Republicans' efforts would have impeded workers' efforts to unionize; just the opposite is true, the Minnesota congressman, who chairs the House Committee on Education and the Workforce, argued.

"With his veto, the president has endorsed an ambush election rule that will stifle employer free speech, cripple worker free choice, and jeopardize the privacy of working families," Kline said.