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 on Tuesday.
Opponents meanwhile are mounting an equally emotional push to keep Obama from obtaining "fast track" negotiating authority.
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At least 60 of the Senate's 100 members must back the measure for it to clear a procedural hurdle Tuesday.
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.
Obama's allies restructured the package. Now they are counting on the 14 Senate Democrats and 48 Republicans who supported fast track in May to do so again.