The final votes from Pennsylvania and Nevada not only changed President Donald Trump's political future — they changed his legal prospects, too. When Trump leaves the Oval Office for the last time, he will face potential liability for any criminal acts he committed while in office, and even before.

Between now and January 20, though, Trump has a lot of power to make it harder for the Justice Department and FBI to follow through on any investigations once he is no longer president.

Until now, the office of the presidency has largely shielded Trump from the prospect of criminal prosecution. Some of this is because of Justice Department policy: despite finding substantial evidence of obstruction of justice in the Russia investigation, for example, special counsel Robert Mueller III declined to charge Trump, citing a 1973 Justice Department policy memo precluding it.

Attorney--General William Barr has further protected Trump, defending the President's actions in public and pressuring prosecutors who might harm the President's interests in private. But without the immunity offered by his position, and with a new attorney-general in charge under Joe Biden's administration, Trump will be accountable to the law like any other citizen.

Trump reportedly planned to fire FBI Director Christopher Wray once the election ended. While that might be emotionally satisfying for the President, we saw when he fired former director James Comey that simply deposing the head of the FBI doesn't actually stop any ongoing investigations. Nevertheless, Trump can take much more concrete steps to protect himself and his family than the simple symbolic one of throwing out an appointee he has decided was disloyal.

The strongest, and broadest, immunity from federal prosecution for Trump would come from a presidential pardon. President Gerald Ford offered Richard Nixon a blanket pardon for any crimes committed while in office, and President George H.W. Bush — with help from Barr, then also the attorney-general — pardoned six people involved in the Iran-Contra affair in 1992, stopping two ongoing prosecutions dating back to the Reagan Administration dead in their tracks. However, since president-elect Biden has categorically stated that he will not pardon Trump, Trump would have to engineer that during the transition.

He has two options. First, he could try to pardon himself. This is a risky move, as whether a self-pardon would be constitutionally valid is an unsettled legal question because no previous president has tried it. Most legal scholars agree that it's not permissible, though, and if a Trump pardon of himself were later challenged and invalidated, he would be back to square one.

Alternatively, Trump could resign at some point before his term ends at noon on January 20, 2021, leaving Vice-President Mike Pence to assume the presidency, however briefly — giving him the plenary power to pardon Trump. Thanks to the precedent that Ford set with Nixon, such a pardon, which Pence could also extend to members of Trump's family, would probably be constitutionally secure if it covered uncharged crimes committed while Trump was in office.

Trump also has a couple of quieter options he could pursue behind the scenes to protect himself. For one, he could get Barr to provide "legal cover" for anything that future prosecutors might want to examine in the form of internal memos written by the Office of Legal Counsel. We already know that Barr has expressed, before taking his current position, legal theories supporting an expansive view of presidential power - including the idea that the President cannot, as a matter of law, obstruct justice.

Barr also determined — without investigation — that Trump's solicitation last year of Ukrainian president Volodymyr Zelensky to investigate Biden was not a "thing of value" that would have constituted a criminal finance violation. The Office of Legal Counsel could incorporate arguments such as these into official legal positions of the Justice Department. While they wouldn't be binding on a future attorney-general, they would undermine future investigations and prosecutions by giving Trump a basis to argue that his actions were legally justified and sanctioned by his own legal advisers.

Trump has a bigger problem with investigations that have already taken place or that may be actively under way. For example, the indictment by federal prosecutors in New York of Trump's former attorney, Michael Cohen, for campaign finance violations stemming from the concealed payment before the 2016 election to a porn actress alleging an affair with the President lists "Individual-1" — who appears to be Trump — as a co-conspirator. It is also possible that federal law enforcement officials there or in other jurisdictions might be actively investigating Trump for tax or bank fraud, given the series of reporting based on Trump's tax returns.

Barr can't make these investigations literally disappear: the investigative documentation that has been gathered is protected from tampering at this stage. However, he can "paper over" the files, adding memos justifying closing the case(s) or calling into doubt the evidence or the assessment of investigators or prosecutors who have worked on them. We have seen him do this already in his attempt to reverse the sentencing recommendation of Trump's campaign adviser, Roger Stone, and in the Justice Department's present attempt to drop the charges against Trump's former National Security Adviser, Michael Flynn.

By "poisoning" the cases in this way, Barr can effectively plant exculpatory material into the record — prosecutors would be legally required to provide these memos to Trump's defence team — and set up future prosecutions to seem as though they are pursuing legally baseless cases because of political bias.

Of course, none of these actions would protect Trump from being prosecuted for state crimes — a very real possibility, given that the New York attorney-general and the Manhattan district attorney are actively investigating Trump's finances and those of the Trump Organisation. They also wouldn't cover any crimes that might be uncovered later — it's hard to imagine that even a blanket, retroactive pardon would legally cover crimes that were unknown at the time it was granted — and a pardon extending to uncharged crimes before Trump took office might be open to legal challenges.

And, of course, it's hard to know what Trump might do during the transition itself, if he feels panicked and decides to get rid of anything he thinks might incriminate him. Any records relating to Trump's official duties as president, even if contained on personal devices or servers, are officially presidential records, and attempts to destroy them would constitute a new host of federal crimes.

In short, Trump won't be in the clear, no matter what he does, once he no longer has the protection of the presidency. This is especially true when we consider that the FBI's counterintelligence interest in Trump will continue even after he leaves office: he will still know highly classified information, so it will be more important than ever for the intelligence community to know whether foreign countries have any leverage they can exploit over the former president. This would provide grounds for further investigation into Trump's, and the Trump Organisation's, financial entanglements abroad.

Still, before Trump leaves office, he has the power to muck up any cases that could be made against him by a future Justice Department — enough to set up Biden to continue as the villain in the same "witch hunt" narrative that Trump has been talking about for the past four years.

• Asha Rangappa is a senior lecturer at the Jackson Institute for Global Affairs at Yale University and a former FBI agent. She is a legal and national security analyst for CNN