By John Whitesides and Luciana Lopez
LAS VEGAS/NEW YORK (Reuters) - Hillary Clinton has veered hard to the left ahead of Tuesday's first Democratic presidential debate, hoping to inoculate herself from criticism by rival Bernie Sanders and woo the union members and liberal activists who have been slow to embrace her.
But in a Democratic race so far featuring few political attacks or policy clashes, Clinton's move to protect her left flank on issues like the Asian trade pact and Keystone oil pipeline could open the door during the nationally televised debate to questions about her sincerity and charges of flip-flopping.
Clinton, the Democratic front-runner, and Sanders, a U.S. senator from Vermont who is her prime challenger, will take part on Tuesday at 8:30 p.m. EDT in the first of six scheduled debates in the race to be the party's nominee in the November 2016 presidential election.
They will be joined by former Maryland Governor Martin O'Malley, former Rhode Island Governor Lincoln Chafee and former U.S. Senator James Webb of Virginia. The showdown will give Sanders his first broad national exposure and offer Clinton a chance to ease the concerns of some Democrats about her.
After two raucous Republican debates that drew big television audiences attracted by the fireworks generated by front-runner Donald Trump, the Democratic encounter, hosted by CNN and to be held in Las Vegas, is likely to be a tamer affair.
It comes at a critical time for Clinton, whose once overwhelming lead among Democrats in opinion polls has slipped amid questions about her use of a private email server instead of a government account when she was secretary of state.
In addition, she faces the threat that Vice President Joe Biden could enter the race - something he has been increasingly urged to do as Clinton's lead falters.
Sanders, a self-described socialist, has excited the party's left wing and generated big crowds with a persistent message of eradicating income inequality and reining in Wall Street.
In response, Clinton took stances on several key issues recently that align her with Sanders. She reversed course to announce her opposition to the 12-nation Trans-Pacific Partnership trade deal that she had praised when she was secretary of state, and she rejected the Keystone XL pipeline that she had said in 2010 she was inclined to approve. Sanders is a longtime opponent of both projects.
Sanders, who has repeatedly refused to directly attack Clinton, signaled over the weekend he would make an issue in the debate of Clinton's tardiness on some of those topics, noting he opposed Keystone and the TPP "from day one."
Clinton, who still faces ambivalence about her candidacy from much of the union rank-and-file, won praise from labor leaders for her opposition to the TPP. Labor has opposed the pact out of fear it would cost manufacturing jobs and weaken environmental laws.
"I don't think she ever had any inclination to back TPP," said R. Thomas Buffenbarger, a Clinton ally who is president of the International Association of Machinists and Aerospace Workers, which has endorsed Clinton.
Still, one labor leader said unionized workers very much want to hear Clinton take her opposition to the TPP even further.
Some subtle policy differences remain between the two top Democratic contenders. Sanders has pushed for what he calls a sensible approach on gun control and voted against the 1993 Brady handgun bill that President Bill Clinton signed into law.
Clinton, who has touted her foreign policy experience as secretary of state, broke with the White House to back a no-fly zone in Syria to give refugees a safe corridor. Sanders opposes it, saying it could be a step toward pulling the United States into the Syrian civil war.
Sanders has discouraged Super PACs from raising funds on his behalf, warning of the influence of corporate money. Clinton is backed by several Super PACs.
Eric Davis, a professor emeritus of political science at Middlebury College in Vermont, expects Sanders "to have a vigorous critique of Hillary Clinton on things like campaign finance," saying he can criticize her for "the way she is financing her campaign and her perceived closeness to Wall Street interests."
(Editing by Caren Bohan and Leslie Adler)