Democrat Hillary Clinton and Republican Donald Trump will face off for the first time on Monday in a presidential debate that could rank as one of the most watched, highly anticipated political showdowns in US history.The tight race for the White House and the unpredictable clash in styles between well-known but polarizing foes has generated wide interest in the potentially pivotal encounter, which comes six weeks before the Nov. 8 election after a campaign that has stretched over more than a year.The gap between the two candidates in recent national opinion polls has narrowed in the past week, with the latest Reuters/Ipsos polling showing Clinton ahead by 4 percentage points, with 41 percent of likely voters.The size of the television-viewing audience was expected to challenge the presidential debate record of 80 million Americans who watched 1980's encounter between Democratic President Jimmy Carter and Republican Ronald Reagan. Some commentators forecast Super Bowl-sized viewership of around 100 million.The 90-minute debate will begin at 9 p.m. (0100 GMT on Tuesday) at Hofstra University on New York state's Long Island. It is the first of three planned presidential debates.The White House race has so far had little discernible effect on the market, but that may soon change.Ahead of the debate, the stock market on Monday showed jitters. A measure of trading volatility, also known as Wall Street's "fear gauge," was up 14 percent in early afternoon trading, its biggest rise in nearly two weeks."Investors are acting extremely nervous with regards to the debate ... and it highlights the fact that the markets are not focusing on the health of the economy, interest rates and geopolitical events," said Robert Pavlik, chief market strategist at Boston Private Wealth.Both Trump and Clinton, who polls show are the least liked White House candidates in modern history, hope to use the debate to erase lingering voter doubts and address campaign-trail weaknesses.The volatile Trump, a New York businessman and former reality television star, will get a chance to show a depth and steadiness worthy of a credible commander in chief, while the cautious Clinton will be able to try to connect directly with voters who do not trust her, strategists said.But Trump, a political newcomer who has often shown more affinity for put-downs than policy, could benefit from lower expectations from voters."There is no question it's a lower bar for Trump," said Dan Schnur, a former Republican strategist who is now a political scientist at the University of Southern California. "He doesn't have to be brilliant, he just can't be too bombastic."The stakes are enormous. The debate comes as polls show Clinton's once-sizable lead over Trump has evaporated amid more questions about her family foundation and her use of a private email server while secretary of state under Democratic President Barack Obama.A Reuters/Ipsos poll released on Monday showed half of America's likely voters would rely on the debates to help them make their choice. More than half, 61 percent, were hoping for a civil debate and were not interested in the bitterness shown on the campaign trail.GRUDGE MATCH Clinton, 68, and Trump, 70, have regularly exchanged sharp insults, raising the prospect of a fiery grudge match. Trump frequently refers to Clinton as "Crooked Hillary" and has called for her jailing for the email controversy. Clinton condemns Trump as temperamentally unfit for the White House.Trump dominated the crowded Republican debates with rapid-fire attacks on his rivals but he has no experience in a one-on-one debate setting that requires more prolonged discussion of issues.Clinton has participated in many one-on-one debates on the national stage with Obama during her unsuccessful 2008 campaign for the Democratic presidential nomination and with US Senator Bernie Sanders in their Democratic nominating race earlier this year.Clinton's camp has done its best to raise the bar for Trump, and in television interviews on Monday both campaigns tried to frame expectations."What we don't want to have is some sort of double standard where Donald Trump can get the most improved award but Hillary Clinton ... is getting judged on the fine points of policy," Clinton campaign manager Robby Mook told NBC News, calling Trump "an entertainer."Trump campaign manager Kellyanne Conway, separately on NBC, said Clinton's vast experience was unlikely to translate onto the debate stage, where Trump held an advantage."He's not going to be robotic and scripted," she said.The role of moderator Lester Holt of NBC News also has come under scrutiny ahead of the debate with the Clinton campaign and her Democratic supporters urging him to correct Trump if he makes false claims.Trump also has tried to influence Holt and the moderators of the other showdowns with Clinton, saying the candidates should be the ones to correct the record.But in a year when outsiders like Trump and Sanders have made a mark, Trump's best argument could be that he is a better agent of change than Clinton, said Scott Reed, a veteran Republican operative who is now chief strategist for the US Chamber of Commerce."He's got to draw the contrast between Trump as the candidate of change and Clinton as the candidate of more of the same," Reed said.