WASHINGTON - President Donald Trump has strongly endorsed the controversial Republican candidate for a Senate seat in a key election this week which has turned into a morality tale in the United States.

Tuesday's (Dec 12) poll for the seat, in the southern state of Alabama vacated by Attorney General Jeff Sessions, was crucial enough for Mr Trump to jump in on Friday with a full-throated endorsement of Roy Moore.

"We cannot afford - this country, the future of this country - cannot afford to lose a seat in the very very close United States Senate," Mr Trump told a roaring crowd in Pensacola, Florida. "We can't afford it folks."

"We need someone in that Senate seat… tough on crime, strong on borders, strong on immigration, building the wall, strengthening our military, continuing our great fight for our veterans. We want jobs, jobs, jobs - so get out and vote for Roy Moore."

The election has become one of the most closely watched as Mr Moore - a 70-year-old gun-wielding conservative Christian and former chief justice of the state Supreme Court - may win despite accusations that he had inappropriately touched a minor, and habitually prowled a mall looking for high school girls when he was a lawyer in his 30s.

The Republican, who once brandished his handgun at a campaign rally to show off his pro-gun credentials, is a whisker ahead in the polls. That he is so close to a win has many pundits puzzled. The seat is considered safely Republican, but Alabama is America's most Christian state and it was assumed evangelicals - a solid support base for Mr Trump and Vice President Mike Pence - would turn against Mr Moore after the revelations.

The Republican Party, which briefly pulled funding for Mr Moore and even considered replacing him, has now rallied round him. A win for his rival, Doug Jones, would give the Democratic Party one of three seats it needs to take back control of the chamber, where Republicans hold a 52-48 majority. The next chance would be the Nov 2018 mid-term elections.

The Washington Post, in a meticulously researched story last month, broke the story about Mr Moore's behaviour when he was a deputy district attorney about 40 years ago. Several women, including Republican supporters, have since emerged to level similar accusations against him. Mr Moore has denied all the accusations.

The Alabama controversy comes amid a wave of allegations of sexual misconduct implicating powerful people and household names in the US. Among them is Democratic Senator Al Franken, who on Friday, said he would resign his seat.

The allegations against Mr Moore also brought back memories of the tape that surfaced during last year's presidential election in which Mr Trump is heard bragging about women allowing him to grope them because he was a celebrity.

In the event, the tape that most political pundits thought would doom Mr Trump's bid for the White House turned out to be a minor dent. That may well be the case with Mr Moore.

Many of his supporters, including women, insist the accusations against Mr Moore were trumped up. According to one poll, 83 percent of Republican women still back him, saying the allegations coming after so many years of silence must be false.

The broader question, however, is whether morality in American politics has fallen to a new low.

"Evangelical Christians who believe they have a higher sense of morality than a lot of other folks are willing to look the other way if the candidate is a Republican," Charles Bullock, professor of political science at the University of Georgia, told The Straits Times.

But the apparent hypocrisy is not the monopoly of Republicans, he noted. Across the country, Democrats who made excuses for former president Bill Clinton's widely documented indiscretions did not see any reason to excuse Mr Moore; and Republicans, who wanted Mr Clinton impeached, support Mr Moore.

"This is part of the polarization," Dr Bullock said. "There is no actual morality about whether this is right or wrong. It depends on where you are on the ideological scale."

Rather than hypocrisy, R. Marie Griffith, professor of religion and politics at Washington University in St Louis, Missouri, sees it as the "extreme politicization of Christianity." People are primed politically to believe or disbelieve a story depending on who was telling it, she said.

"It's not that people condone what Roy Moore did, but somehow he seems to be preferable to Doug Jones," she told The Sunday Times in a phone interview.

The professor, whose latest book "Moral Combat: How Sex Divided American Christians and Fractured American Politics" is due out this month, said patriarchy and expectations of gender were also part of the mix of reasons.

But she added : "This really needs some deep explanation of why evangelicals, in particular, and other conservatives who have been so strict about sexual morality and uprightness for so long, have seemed to just sort of shuck that in favour of something else."

"I don't know if we are lower than we have ever been, but we certainly seem to be going lower and lower right now."