The U.S. Senate votes today to allow time-limited debate on an important bill to give President Barack Obama and his successors the authority they need to negotiate trade agreements without second-guessing by Congress.

The procedural measure, which requires a 60-vote supermajority, is critical for legislation to let the executive branch reach agreements that will promote the future health of the U.S. economy. Congress would still have the right to vote for or against each trade deal on an up-or-down basis.

But other nations won’t make those agreements with America if they think their efforts will be overridden on a piecemeal basis by a protectionist Congress imposing amendments of its own.

One such deal, the Trans-Pacific Partnership, is all but complete, and at least two other major trade agreements are pending. Combined, they cover over a trillion dollars in annual trade and would reduce barriers to U.S. exports.

The Senate has already passed Trade Promotion Authority (TPA) by a healthy 62-37 majority, with 14 Democrats joining 48 Republicans to support the measure requested by President Obama. But that bill also contained language extending and enriching training programs for workers displaced by foreign competition, a measure usually strongly supported by labor unions.

The Senate has to come back to the question because House Democrats, taking their marching orders from the AFL-CIO, surprisingly voted against that Trade Adjustment Assistance in an effort to block TPA. They were unable, however, to block the House from adopting a stand-alone TPA bill.

Now the Senate plans to take up each measure separately. Senate Republicans have agreed to sweeten the training bill by making it easier for the steel industry to challenge imports it claims are unfairly priced. In the House vote on June 12, President Obama was embarrassed by Minority Leader Nancy Pelosi’s rejection of his plea for support. The president has also been abandoned on this issue by Hillary Clinton, who had praised and promoted the Trans Pacific Partnership while serving as Secretary of State.

So now the president must make his strongest efforts to keep the votes of Senate Democrats who supported him last time but face newly aggressive pressure from Big Labor to switch their positions.

If the Senate adopts the House version of TPA, as it should, it will go to the White House for Mr. Obama’s signature no matter what happens to Trade Adjustment Assistance.

That wouldn’t eliminate proper congressional input on trade deals. Again, federal lawmakers would retain the right scuttle such accords via up-or-down votes.

But allowing Congress to make numerous amendments to trade agreements doesn’t just undermine rightful executive-branch authority.

It effectively eliminates a president’s ability to negotiate beneficial trade deals with other nations.

That puts high stakes on the Senate moving the president’s trade authority forward today.