WASHINGTON (Reuters) - Hillary Clinton’s campaign is confronting an emerging risk to her presidential ambitions - if Donald Trump continues to trail her in opinion polls many Democrats may simply stay at home on Election Day. Without enough popular support, Clinton would enter the White House lacking the political capital she would need to drive through her agenda. In the worst-case scenario it could cost her the presidency if Republicans turn out in big numbers on Nov. 8.      Clinton, the Democratic nominee, has spent much of her campaign sounding the alarm over the prospect of a President Trump. She has struggled to lay out a compelling vision for her presidency and has failed to excite key constituencies, including millennials, minority voters and liberal Democrats. Opinion polls show that many voters are backing Clinton primarily to stop Trump, the Republican nominee, from getting into the White House. If they believe he has no hope of winning, then what would their motivation be to turn up at the polls?  In a recent Reuters/Ipsos poll about half of all Clinton supporters said they were backing her to keep Trump from winning. By contrast, just 36.5 percent said it was because of Clinton’s policies and just 12.6 percent said it was because they like her personally.  “Turnout is correlated with levels of competition,” said Michael McDonald, an elections expert at the University of Florida. “The higher the competition, the higher the turnout.” The young Americans, blacks, Latinos, and low-income voters who make up much of the Democratic base often need to feel motivated by a particular candidate or issue to turn out, McDonald said, as was the case with President Barack Obama’s candidacy in 2008.  Clinton’s campaign has long worried about voter complacency and has at every turn pushed the notion that the race is close and that Trump is unfit to be president. With her lead growing, that task grows more difficult. A Reuters/Ipsos 50-state survey (carried out before Friday’s release of a video tape in which Trump makes vulgar remarks about women) gave the Democratic nominee a 95 percent chance of winning the election. An NBC/Wall Street Journal poll on Monday showed Clinton with an 11-point lead nationally over Trump. Low Democratic voter turnout could leave Trump an opening in swing states. And should Clinton win the election, a slim margin of victory could compound the challenge she will face in trying to govern a deeply divided nation.  Clinton’s campaign, however, will be able to rely on an extensive and well-funded voter mobilization effort, one that is expected to give her an edge over Trump’s smaller organization.  The Clinton campaign insisted on Monday the race will remain tight. It sent out a new fundraising pitch to supporters, contending that Trump is “an authoritarian threat” for saying at Sunday’s presidential debate that she would be in jail if he was president.  Clinton must also contend with anger among liberal Democrats over leaked excerpts of paid speeches she made to banks and big business. The excerpts appeared to confirm their fears about her support for global trade and tendency to cozy up to Wall Street. Some liberals have also been waiting for Clinton to make a more positive case for her own presidency. “This election cannot be just a referendum on Donald Trump,” said Arun Chaudhury, creative director of Revolution Messaging, a left-leaning consulting firm that oversaw the online media operation of former Clinton rival, Senator Bernie Sanders.  Clinton’s central message, he said, has been that “everyone has to step up and stop Donald Trump from being president, not step up and make Hillary Clinton president.”  “The best campaign messages are comparative in nature,” said Ben Turchin, a Democratic pollster who worked for Sanders’ campaign. “She can win by a bigger margin by giving a little more of an affirmative case for her presidency.” While Clinton frequently goes on the attack against Trump, calling him racist, sexist and dangerous, her campaign insists it has been trying to get a positive, policy-oriented message out. “It is hard in this campaign when you’re running against him and he generates so much controversy and therefore headlines,” Jennifer Palmieri, Clinton’s communications director, told Reuters. “It’s hard to break through on any one day, and that’s why we just have to keep at it.” The two candidates’ central campaign slogans reflect their differing appeals to the electorate. Where Trump’s is the change-oriented “Make America Great Again,” Clinton’s is a more stolid “Stronger Together,” which speaks to rallying existing Democratic voters around her candidacy - and is a harder sell.  Clinton’s campaign seems to have recognized the need for some adjustments to its message.  Since the presidential race intensified last month, Clinton has returned to the style of campaigning that helped her win early states in the Democratic nominating contests, holding smallish events focused on issues of most concern to core Democratic constituencies such as women and young voters.  Turchin, the former Sanders pollster, said Clinton’s efforts at fashioning a positive message were improving, although she is still having difficulty attracting the support of 18-to-34 year-old voters, among others.  “You’ve got to make the hard case over and over again,” he said. “She’s got to convince people she shares their values.” 