(Note that this article became highly important after the report today that President Donald Trump and his wife have Covid-19)
Therefore should the worst ever happen to President and Vice President, she could theoretically take over at the White House.
The next scenario where the next in line would take over is if a new president and vice-president has not been inaugurated by the end of Trump's term at noon on 20 January.
Some observers say this could happen if election results are delayed.
But should this happen Pelosi would NOT be second in line — because she is herself up for 2020 election.
Therefore the next in line would be the most senior Senator, Chuck Grassley, R-Iowa, who is not up for election in the November poll.
What does the 20th amendment to the U.S. Constitution say? The 20th amendment of the US Constitution says the president and vice president must leave office if no successor has been chosen by the end of their term on January 20.
Judge Andrew Napolitano, a Fox News senior legal commentator, said: "President Trump's term ends at precisely noon on January 20, 2021.
"If the Electoral College has not yet named a successor, presumably either Donald Trump or Joe Biden, then whoever is the Speaker of the House would become the acting president of the United States.
"If the Democrats retain their majority in the House, and it appears likely that they will, but if they do, and if they choose Mrs Pelosi, knowing at that point that they're choosing the president of the United States, then it would be she.
"Stated simply, whoever is the sitting speaker of the United States, would become president at noon on January 20, 2021, if the Electoral College has failed to elect someone."
What has Nancy Pelosi said about the prospect of becoming President? In July Pelosi noted she is "in line to the presidency" should the worst happen to Trump or Pence and she had her "regular continuation of government briefing" detailing the process.
She said: "I say to them [the briefers], "This is never going to happen.' God willing, it never will. But there is a process.
"The presidency is the presidency. It's not geography or location."
Trump and "crazy Pelosi" have frequently clashed throughout his tenure as president.