WASHINGTON (AP) — Republicans in Congress launched bids Thursday to nullify Obama administration rules that would speed up union elections and set the first national air pollution standards for toxic mercury pollution from the nation's power plants.

The rarely used tactic, known as a resolution of disapproval, requires a simple majority for passage. Both have only a small chance of clearing the Senate, but a vote would force some Democrats to take a public stand on two volatile issues in an election year.

The power plant resolution was introduced Thursday by Sen. James Inhofe of Oklahoma. It could force some Democrats to choose between slashing pollution or eliminating jobs, especially those from states with older coal-fired power plants at risk of shutting down because of the Environmental Protection Agency rule. Some states also already control mercury pollution from power plants, giving some lawmakers a reason for voting against a national standard.

"Over the past year, more than a dozen Senate Democrats have claimed that they want to stop EPA's destructive agenda, yet when the time comes, they hide behind alternative bills they know will never pass," Inhofe said in a statement. "It's time for the Senate to do its job and stop this regulatory nightmare."

An earlier effort to block an EPA regulation in the Senate using the same tactic failed. In November, the Senate defeated a resolution from Sen. Rand Paul, R-Ky., to block a rule aimed at curbing pollution from power plants that contributes to poor air quality downwind. While no resolution has been introduced in the Republican-controlled House, that chamber has already passed legislation to block the regulation targeted by Inhofe's effort.

GOP leaders in the House and Senate also filed resolutions Thursday to block new National Labor Relations Board rules that make it easier for unions to hold elections after employees at a work site have gathered enough signatures to form a union.

The labor board approved the rules in December over the vigorous objections of business groups that claim it doesn't give company managers enough time to dissuade workers from voting to unionize. The NLRB and labor advocates say current rules allow too many frivolous legal challenges to hold up an election.

The U.S. Chamber of Commerce and other business groups have filed federal lawsuits challenging the rules, which are set to take effect April 30.

Senate Republicans need only 30 signatures to force a vote on the measures, bypassing any Democratic attempts at obstruction. But even if the resolutions pass, they will be subject to a likely presidential veto.

The tactic has been used successfully 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.