WASHINGTON (NYTIMES) - US President Donald Trump is on the defensive in three red states he carried in 2016, narrowly trailing Joe Biden in Iowa and battling to stay ahead of him in Georgia and Texas, as Trump continues to face a wall of opposition from women that has also endangered his party's control of the Senate, according to a poll conducted by The New York Times and Siena College.

Trump's vulnerability even in conservative-leaning states underscores just how precarious his political position is, less than six weeks before Election Day.

While he and Biden are competing aggressively for traditional swing states like Wisconsin, Pennsylvania and Florida, the poll suggests that Biden has assembled a coalition that could be formidable enough to jeopardise Trump in historically Republican parts of the South and Midwest.

A yawning gender gap in all three states is working in Biden's favour, with the former vice-president making inroads into conservative territory with strong support from women.

In Iowa, where Biden is ahead of Trump, 45 per cent to 42 per cent, he is up among women by 14 percentage points. Men favour Trump by 8 points.

In Georgia, where the two candidates are tied at 45 per cent, Biden leads among women by 10 points. Trump is ahead with men by a similar margin of 11 percentage points.

Trump's large advantage among men in Texas is enough to give him a small advantage there, 46 per cent to 43 per cent. Men prefer the president to his Democratic challenger by 16 points, while women favour Biden by an 8-point margin.

There was a significant gender gap in the 2016 election, too, but at that time it tilted towards Trump because men supported him so heavily, according to exit polls. In the Times poll, Biden sharply narrowed Trump's advantage with men while improving on Hillary Clinton's 2016 lead with women in Texas and Iowa.

In Georgia, Biden's lead with women essentially matched Clinton's final advantage in the 2016 race. But where Trump carried Georgia men by 23 points four years ago, he was ahead by about half that margin with men in the state in the Times poll.

The overwhelming majority of voters - about 9 in every 10 in all three states - say they have definitely made up their minds about whom to vote for, leaving relatively little room for late developments to shift the overarching shape of the race.

The poll, conducted by phone among likely voters from Sept 16-22, had a margin of sampling error of 4 percentage points for Texas and 5 in Iowa and Georgia.

Trump's tenuous hold on some of the largest red states in the country has presented Biden with unexpected political opportunities and stirred debate among Democrats about how aggressively to contest states far outside the traditional presidential battleground.

Biden has made efforts so far in a few states that voted emphatically for Trump four years ago, including Georgia and Iowa, but has resisted pressure to compete for Texas, a huge and complicated state that Democrats believe is unlikely to furnish the decisive 270th Electoral College vote.

But the presence of competitive Senate races in many of those states has been a powerful enticement to Democrats, including Biden.

The lopsided gender dynamics of the presidential contest extend to Senate races in Iowa, Georgia and Texas, with Republican incumbents facing strong challenges from Democratic candidates favoured heavily by women. The gender gap is pronounced even in Iowa, where both Senate candidates are women. The Democratic challenger, Theresa Greenfield, has a 2-point lead over Senator Joni Ernst and an 11-point advantage with women.

The poll partly coincided with Trump's announcement that he would make a new Supreme Court nomination to fill the vacancy created by the death of Justice Ruth Bader Ginsburg, but it was not clear from the survey whether voters had a particularly strong reaction to that possibility.

Reflecting the conservative tilt of the states polled, Trump and his party are in better shape than in most of the others recently polled by The Times, and he may ultimately carry all of them. The president's approval rating is in positive territory in Texas, and voters are almost evenly split in Iowa and Georgia. That is markedly stronger than Trump's standing in core swing states like Wisconsin and Arizona.

Trump has maintained an enduring advantage over Biden on economic issues, and that extends to all three states in the Times poll. And where voters elsewhere have heavily favored Biden over Trump on the issue of managing the coronavirus pandemic, voters in Texas and Georgia are closely divided on that score. Biden still holds a sizable advantage on the issue in Iowa.

In Georgia and Texas, the election is also split along racial lines. Trump is winning about two-thirds of white voters in both Georgia and Texas, while Biden leads by enormous margins with Black voters in both states. Hispanic voters in Texas favour Biden by 25 points, 57 per cent to 32 per cent.

Still, many of the same voters, in heavily white Iowa and two traditionally conservative Southern states, are not as dismissive of systemic racism as Trump is. In each state, half or more of those surveyed found racism in the country's criminal justice system to be a bigger problem than rioting.

And as with The Times surveys of other competitive states from earlier this month, voters expressed little confidence in Trump's ability to heal the country.