Donald Trump is continuing his attempts to overturn the result of the US presidential election, but he has been dealt a series of legal blows and time is running out. Electors from each state meet on 14 December to formally nominate the next president. So what legal cases remain? Series of defeats so far in PennsylvaniaThis is where Mr Trump has launched most of his challenges, and although Mr Biden has already been certified the winner with a margin of over 80,000 votes, the president has not given up attempts to overturn the result. Mr Trump's legal team has claimed voters in Democrat-leaning areas were given more of an opportunity to correct mistakes on their postal ballots than elsewhere. When they initially lost the case, his lawyers took it to a federal appeals court. It was rejected there too, with the judge saying: "Charges of unfairness are serious. But calling an election unfair does not make it so. Charges require specific allegations and then proof. We have neither here." Mr Trump's team said they would be taking the case to the US Supreme Court. It's uncertain if the court would hear any such appeal, and legal experts say even if it did, the case would have little chance of success. In the same lawsuit, President Trump's team has also alleged that more than 680,000 postal ballots were counted without proper oversight from poll watchers. This follows a legal tussle over where these observers were allowed to position themselves during the count - the Trump team arguing that poll watchers were prevented from seeing what was going on. A lawsuit to stop the count in the city of Philadelphia was rejected. Election officials there insisted they had acted properly and appealed to the Pennsylvania Supreme Court, which on 17 November ruled officials in Philadelphia didn't violate state law when restricting how close poll watchers could be to the processing of postal ballots. Another outstanding challenge in Pennsylvania centres on the state's decision to count ballots that were postmarked by election day, but arrived up to three days later. Republicans argued that all postal ballots received after election day should be disqualified. The US Court of Appeals rejected the case, but the Supreme Court could still hear the Republicans' appeal. Pennsylvania state officials estimate that about 10,000 ballots were received in the three days after election day. They say these ballots are being kept on one side, in the light of on-going legal challenges. The courts in Pennsylvania have also rejected a string of legal challenges by the Trump campaign based on around 9,000 postal ballots they said lacked information, such as the date the ballot was cast or the voter's address. The Trump legal team did have one a small victory over how long voters should be given to provide proof of identification if it was missing or unclear on their postal ballots. The deadline was 12 November, but following a court ruling this was reduced by three days.