WASHINGTON (NYTIMES) - The Postal Service has continued to struggle in the days before the election to meet its standards for on-time delivery of mail ballots despite court orders intended to speed its operations.

The percentage of ballots delivered to election officials nationwide in one to three days has hovered in the low to mid-90s in the past week.

But portions of swing states like Pennsylvania and Michigan have seen significantly lower rates in the closing days of the campaign, leaving some voting rights groups concerned about ballots being mailed in time to be counted.

According to an estimate by the United States Elections Project, about 29.6 million mail-in ballots are still outstanding, although it is not clear how many of those were never sent back or reflected decisions by voters to cast their ballots in person.

Judge Emmet Sullivan of the US District Court for the District of Columbia ordered on Sunday evening (Nov 1) that the Postal Service again emphasise to its workforce the need for "extraordinary measures" to ensure that every possible ballot was delivered by the cutoff time on Election Day, or Monday in Louisiana.

Judge Sullivan instructed the agency to clear and process all local ballots for delivery to election officials by no later than the next morning until at least Saturday. The agency must also use its Express Mail network to expedite certain ballots.

Pennsylvania, whose electoral votes could clinch a victory for either candidate, has experienced some of the most significant mail delays in the delivery of ballots.

In the district covering the centre of the state, daily on-time ballot delivery rates over the past 10 days have ranged from 83.28 per cent to as low as 56.21 per cent.

The Philadelphia metropolitan area has seen better on-time delivery rates during the same period - between 78.01 per cent and 96.7 per cent- along with the western Pennsylvania district.

The Postal Service has blamed the effects of the coronavirus and a lack of available employees for service issues in central Pennsylvania.

In a court filing on Sunday, the agency said it was moving employees to its processing plant, maximising overtime and hiring new employees to address concerns about mail ballots.

Figures show problems in Michigan as well. Over the past week, Detroit has seen its on-time rates vary from below 50 per cent to 80 per cent.

In the Greater Michigan district, covering the rest of the state, the rate varied from 55 per cent to 96 per cent.

However, in that district - as in others around the country, like in North Carolina - the Postal Service said in court documents that the figures did not include ballots that were processed in local post offices and "delivered the same day with a near 100 per cent success rate".

The agency has also argued that daily delivery rates submitted to the courts do not accurately reflect performance, as the scores fluctuate based on day and mail volume.

However, a Postal Service spokesman said he could not provide weekly figures by region.

The Postal Service has blamed the pandemic and labour shortages for some of the poor rates.

As coronavirus cases surge across the country, the Detroit district reported just 78 per cent of its employees were available to work, while central Pennsylvania reported 84 percent, according to a court filing on Friday (Oct 30).

"This has been one of the things that I've been really worried about this year," said Ms Tammy Patrick, a former Arizona elections administrator and a senior adviser at Democracy Fund, a research and advocacy group.

"When you get closer and closer to Election Day, your options tend to run out."

Mr Keith Combs, the president of the Detroit chapter of the American Postal Workers Union, said the post office was still recovering from operational cutbacks imposed this summer by Mr Louis DeJoy, the postmaster-general, while it continued to deal with a shortage of employees.

Backups in mail delivery were better than they once were, but customers were still experiencing delays, Mr Combs said.

The Postal Service has been careful to try to shield ballots from the slowdowns, but some might slip through the cracks, he added.

Michigan and Pennsylvania are far from the only places to report problems.

Lakeland, the Postal Service district that covers most of Wisconsin and northern Illinois, has consistently fallen below the national average, along with Atlanta.

The Gulf Atlantic district, which includes parts of Georgia and Florida, has also performed below the national average most days this week.

Other swing states have not experienced delays that are as widespread.

Certain postal districts in Florida, including Suncoast and South Florida, have mostly had ballot delivery to election officials at rates above 90 per cent this week.

With the exception of Friday and Saturday, the rate of on-time mail delivery in Arizona has stayed above 97 per cent.