Donald Trump is expected to launch his re-election campaign on Tuesday, kicking off an age-old battle that most sitting US presidents wage for four more years of power.Recent history suggests that the 2020 advantage goes to Trump - although Democrats have fielded an extraordinarily diverse group of candidates pursuing the nomination, and the brash New York billionaire has been saddled with scandal and calls for his impeachment.Can Democrats overcome Trump's dominant position and reclaim the White House? Here is a look at how the power of incumbency impacts presidential politics.Is past prologue?Of America's 44 previous presidents, just 15 served two full consecutive terms. But recent elections have especially favoured sitting presidents.Since World War II, only three incumbent candidates have failed to win re-election. The three presidents prior to Trump - Bill Clinton, George W Bush and Barack Obama - all won two terms.AdvantagesModern-day presidents are difficult to unseat. They enjoy massive name recognition, have mega-millions of campaign dollars at their disposal and control the instruments of government - and in Trump's case, social media - that keep a president in the spotlight.Trump is a master at all three. Even before entering politics he was a household name thanks to his reality TV show The Apprentice and his reputation as a real estate tycoon.Earlier this year his campaign released impressive fundraising figures that showed Trump easily outpacing individual Democratic rivals.Polling has not been kind to Trump. But poll numbers have often exaggerated the prospects for challengers, as was the case with Al Gore against president George W. Bush in 2000 and Mitt Romney vs president Obama in 2012.Trump's unprecedented presidential use of Twitter has served to boost his public profile. But the spotlight is a double-edged sword, as opponents can easily document his tweeted falsehoods, and he faces very public demands for the release of his tax returns.Disadvantages While Trump's three recent predecessors won re-election, there is no guarantee that history will repeat itself.When parties fail to coalesce around their president during re-election bids, primary challenges emerge. They proved fatal for Republican Gerald Ford, who only barely saw off a challenge by conservative Ronald Reagan before losing to Jimmy Carter in the 1976 election.Carter's bitter battle with fellow Democrat senator Edward Kennedy left him weakened, and he got walloped by Reagan in 1980.An unpopular war in Vietnam proved to be the undoing of Lyndon Johnson, who scrapped his own 1968 re-election bid.Poor economic figures like rising unemployment sank the elder Bush's 1992 re-election. He was also damaged by the populist independent candidacy of businessman Ross Perot, whose campaign Bush blamed for his loss to Clinton.Seventeen months from the election, Trump suffers from few of the disadvantages that ruined incumbents in recent decades.There is no major foreign war and the economy is strong, while his only primary challenger, William Weld, has barely registered on the national radar.