WASHINGTON (AP) — Backers of President Barack Obama's trade agenda are imploring key senators to stand by their previous votes when they revisit the issue in a showdown set for Tuesday.

Opponents meanwhile are mounting an equally emotional push to keep Obama from obtaining "fast track" authority to negotiate trade agreements with Pacific Rim countries and others.

At least 60 of the Senate's 100 members must back the measure for it to clear a procedural hurdle Tuesday and complete a near-miraculous resurrection of the White House priority. In a May 21 vote, 62 senators backed fast track, but they didn't expect it to return to their chamber.

The House revived the fast track legislation last week after Democrats initially derailed it in a complicated legislative package. Republican leaders — who support Obama on trade while most of his fellow Democrats oppose him — restructured the package and then passed the key elements, with only 28 House Democrats.

Obama's allies now are counting on the 14 Senate Democrats and 48 Republicans who supported fast track in May to do so again. Lawmakers generally dislike voting both yes and no on a contentious issue, figuring it's better to draw the enmity of only one side.

Senate Majority Leader Mitch McConnell, R-Ky., urged senators to stick with their May positions.

"We shouldn't let this opportunity for a significant bipartisan achievement slip past us," McConnell said Monday. "If we simply vote the same way we just did a couple weeks ago, we won't."

Anti-free-trade groups are employing ads, phone banks and other tools to defeat Obama's trade agenda. An AFL-CIO ad warns that the legislation includes "no training for displaced workers" who lose their jobs to international trade.

Such aid, known as trade adjustment assistance, was linked to fast track in the original packaging. After House Democrats, at the AFL-CIO's urging, derailed the whole package by killing the training component, Obama's allies agreed to separate the two issues and try again.

The proponents on Tuesday can afford to lose only two or three senators from the May tally. A chief worry is that a few Democrats might switch from yes to no because they're frustrated that the Republican-led Congress hasn't cleared the way to reauthorize the Export-Import Bank.

It's a priority, for instance, for Sen. Maria Cantwell, D-Wash. Her office said Monday she was keeping her options open on fast track.

"I know Maria is very upset, and I don't blame her," Sen. Bill Nelson, a pro-trade Florida Democrat, told reporters.

Previous presidents have enjoyed fast track authority, which lets them negotiate trade deals that Congress can ratify or reject, but not change. If Obama obtains the authority, he's expected to ask Congress to approve the Trans-Pacific Partnership with Japan, Mexico, Canada and several other countries.

Unions strongly oppose the deal, saying it will cost U.S. jobs.