Victory and concession speeches carry no legal weight.
A candidate can concede or declare victory at any point during the vote counting when the tally looks bad/good for them, but the actual winner is the person who has the most votes when the counting stops.
In recent elections, some Republicans have petulantly refused to concede. Roy Moore, for example, never conceded, but that made no difference to the fact that he lost.
Concession is a mark of civility and class, like calling the victor to congratulate him or her, thanking supporters, and speaking warmly of hope for bipartisanship. It is crass, petty, immature, and selfish for the loser not to concede gracefully. But Republicans think they have an entitlement to rule, and that no non-Republican winner is legitimate, even in a landslide.
As noted above, claims of victory or defeat have no legal significance, but they do have psychological significance. If the legitimate winner wrongly concedes, as Gore did in 2000, it has the effect of making any dispute about the results seem illegitimate to the public.
In the imminent 2020 election, a record number of ballots will still need to be counted after election night. Federal law gives states 35 days to count them, although some individual state laws mandate a shorter timeframe. Too many ballots will remain to be counted for us to know for certain who has won the presidential race and many down-ballot races on election night, unless the victory is by such a huge margin that it exceeds the number of ballots yet to be counted.
So, expect a harrowing election night, but do not expect to go to bed knowing the winner of most races. Nor should you expect many candidates of either party to concede that night. Do expect Trump to declare victory no matter what the vote tally shows on the night. This is tactical; his Orwellian strategy is to try to make people believe falsehoods by simply stating them repeatedly as if the mere repetition of lies makes them true.
It is anticipated that there will be a "red mirage" on election night, with more Republicans going to the polls on the day and more Democrats voting by mail, leading to a false sense of Republicans leading in many races until all the votes are counted.
Expect Republicans in general, and Trump in particular, to argue that the count should be stopped and mail-in ballots should not be counted. In anticipation of this, some Republican-controlled counties are setting aside mail-in ballots to count after Election Day with the expectation that they may not be counted at all.
If the red mirage does not occur, and Trump, and other Republican candidates, are behind in important races, expect them to abruptly change their tune and argue vociferously that every ballot must be counted and the election is not over until they are. Remember that one group of ballots not counted until after election night is military ballots, which skew heavily Republican.
The Republican stance on counting ballots will depend on how they are doing in their races, but either way they will pretend it is on principle. Both parties will find this risible, but, just like with the SCOTUS seat, Republicans do not care about hypocrisy when it benefits their side. Not a single Republican believed the argument that filling Ginsburg's seat was not hypocritical after their refusal to fill Scalia's because this time the same party controlled both the Senate and the White House, They simply did not care. They had power that they could use to benefit their own side, so they did.
Given the prevailing belief among Republicans that mail-in ballots will contain more votes for Democrats, they have been trying to slow down the mail so that ballots will not be received by Election Day. Some states require ballots to be postmarked by Election Day, but others require them to be received by Election Day. Alaska and Ohio have the most generous deadlines, accepting ballots up to 10 days after Election Day. By slowing their transit, the Republicans can prevent ballots not received by Election Day in states that require it from being counted. In response to the Republican mail tampering, some states have scrambled to change their laws to allow counting of ballots received after Election Day. These changes have been challenged in court by Republicans, with mixed results, depending upon whether the change was initiated by the state legislature or the state courts, and other factors. Another strike against the already appalling record of 2020 may be that it proves to be the first U.S. presidential election when mail tampering is such an effective voter suppression tactic that it changes the outcome.
The only guarantee of a Democratic victory is such a large majority of votes for Biden on Election Day that the uncounted ballots will not change the outcome. If that transpires, it is unlikely to prevent Trump from challenging the results. He, and the Republicans, can still harness electoral votes to override the popular vote.
How? Shockingly easily, as it turns out: All states but two give all of their electoral votes to the candidate who wins the popular vote in the state. (Nebraska and Maine divide them proportionally based on percentage of popular vote.) But they do not have to do this; the state legislature is at liberty to ignore the popular vote and give the state's electoral votes to the candidate of their choice.
That sounds radical, but the legal barriers are surprisingly thin. The Constitution allows states to choose electors 'in such Manner as the Legislature thereof may direct'. All states currently have laws directing their legislatures to allocate electors based on the popular vote, but these laws can be changed. A state with a Republican-controlled legislature and a Republican governor (of which there are currently 21) can do it easily; if the governor is a Democrat, the legislature will have to have a sufficient number of Republicans to override the governor's veto. If a state has both a Republican governor and a Republican-controlled legislature, it is likely to be a red state in which the popular vote favours Trump anyway, but there are a few Republican-controlled battleground states that could go to Biden (e.g. Arizona, Florida, Georgia, Ohio, Texas).
If Trump is in need of electoral votes, states with Republican-controlled legislatures and Republican governors can simply ignore the state's popular vote and allocate all of their state's electoral votes to Trump. This manoeuvre could give Trump a second term even if he loses the popular vote by a wide margin, and even if he is unable to halt counting of mail-in ballots or otherwise invalidate legal Democratic votes.
Of the many nefarious schemes the Republicans are plotting to secure victory, this strikes me as the easiest and most likely for them to pursue unless the electoral vote is so close they they only need to stop the vote count in a few states to reach 270, in which case they have the federal courts in their pocket to enable them to do that.