WASHINGTON - As the Senate prepared to vote on White House-endorsed trade promotion authority Tuesday afternoon, the rift  between President Obama and his usual Democratic supporters over trade remained wide. The president has formed an unusual alliance with Republicans and put part of his legacy on the line.

Trade promotion authority allows the Senate and House an up-or-down vote on free trade agreements within a fixed time period. but does not include the ability to amend those agreements. Trade promotion authority, also known as fast track, is considered essential for the passage of any significant trade agreement.

The U.S.. has two major trade agreements under negotiation.  The Trans Pacific Partnership would set trade rules for a dozen countries bordering the Pacific Ocean. Other countries could be added later. The agreement will cover 40 percent of the world's economy. Also at stake in the trade promotion authority vote is a trade agreement with Western Europe called the Trans-Atlantic Trade and Investment Partnership.

Democratic Sen. Al Franken of Minnesota has said he cannot support fast track without more protections for American workers. Sen. Amy Klobuchar has not taken a position, but has said she would look at any trade promotion bill to see “how it addresses important issues like intellectual property, currency manipulation, and transparency.”