 -- Diminished enthusiasm and a high level of negative support are undermining Donald Trump's candidacy in the closing stretch of the 2016 campaign, the ABC News election tracking poll finds, while Hillary Clinton has improved on both these measures.

Fifty-six percent of Clinton's backers in the national survey — a new high — say they're voting mainly to support her rather than to oppose Trump. By contrast, 54 percent of Trump voters are mainly motivated by opposition to Clinton, not support for him.

See PDF with full results, charts and tables here.

Affirmative support can be a stronger motivator to vote, and Clinton's has gained 9 points from its low just before the party conventions in July. Trump's affirmative support, by contrast, has been essentially flat in the same period.

Levels of enthusiasm for the candidates, while similar overall, have also followed different trajectories. Fifty-two percent of Clinton's supporters now describe themselves as very enthusiastic about their choice — the highest level to date and up sharply from 36 percent in early September. Among Trump supporters, 49 percent are strongly enthusiastic; he peaked on this measure in late September.

The result of these trends is that Trump's 12-point advantage in strong enthusiasm just after Labor Day is now a (nonsignificant) 3-point deficit to Clinton. In ABC News/Washington Post polling since 2000, the candidate with more strongly enthusiastic support has won.

As first reported Sunday, the first three nights of the 2016 ABC News tracking poll, produced for ABC by Langer Research Associates, found Clinton with her biggest lead of the campaign in vote preference, 50 to 38 percent over Trump among likely voters. That will be updated in tomorrow's report.

In one noteworthy result, 5 percent of likely voters in the survey indicate that they have already voted, a number in line with estimates reported by turnout expert Michael McDonald of the University of Florida. Total early and absentee voting is expected to reach more than a third of votes cast — a record.

October leads of this magnitude have been seen often in ABC/Post pre-election polls: 11 points for Barack Obama over John McCain in 2008, 19 for Bill Clinton versus Bob Dole in 1996, 13 for Clinton over George Bush in 1992, 13 for Bush versus Mike Dukakis in 1988 and 18 points for Ronald Reagan over Walter Mondale in 1984. With the exception of 1984, the final outcome narrowed in each case, but the leader won.

Groups

There are differences among each candidate's supporters in strong enthusiasm. Fifty-seven percent of women who favor Clinton are very enthusiastic about it, compared with 44 percent of men. Among those age 50 and up, 61 percent are very enthusiastic, compared with 41 percent of her supporters younger than 50. Liberals who back Clinton are more strongly enthusiastic than moderates, 59 versus 45 percent.

In Trump's case, strong enthusiasm peaks at 59 percent among his very conservative supporters, dropping to 43 percent among somewhat conservatives and moderates.

In terms of affirmative and negative support, liberals who back Clinton are more for her than against Trump, 62 to 35 percent, while moderates divide evenly. By contrast, 57 percent of conservatives who back Trump say they're mostly opposing Clinton, compared with a split among moderates. In no group, save rural voters, does a majority back Trump affirmatively.

Methodology

This ABC News poll was conducted by landline and cellular telephone Oct. 20 to 22, 2016, in English and Spanish, among a random national sample of 874 likely voters. Results have a margin of sampling error of 3.5 points, including the design effect. Partisan divisions are 36-27-31 percent, Democrats-Republicans-independents.

The survey was produced for ABC News by Langer Research Associates of New York City, with sampling, data collection and tabulation by Abt-SRBI of New York City. See details on the survey's methodology here.