Republicans controlling the Senate have moved to try to reverse a new National Labor Relations Board rule that would shorten the time between a union's request for representation and the vote on whether to certify a union.
The White House promised to veto the measure just before the Senate's 53-45 party-line vote.
Senate Republicans are employing a little-used process to that allows Congress to pass a bill to try to reverse agency regulations without the threat of a Democratic filibuster. A final vote is likely on Wednesday.
But the promised veto by President Obama would kill the effort.
The new rule would also give unions the email addresses and telephone numbers of workers to more easily communicate with them in advance of a vote.