It's unusual for an incumbent to be challenged by his own party in a presidential election. But when the 2020 Election rolls around, President Trump could face some stiff competition from within the GOP after running a highly volatile 2016 campaign and after losing the popular vote by almost 3 million.

Trump, who makes his first joint address to Congress on Tuesday night, has often defied presidential protocol and some Republicans may feel they have a real shot at capturing the nomination despite his overwhelming name recognition. Should Trump face a battle to remain the nominee in 2020 it could spell doom for him and the party. The last two one-term presidents, Jimmy Carter and George H.W. Bush, each appeared vulnerable in the general election because of a lack of confidence from their base.

But Trump may embrace the role. Even after his Nov. 8 victory over Democrat Hillary Clinton, Trump kept making speeches a part of a "thank you" tour, as he spread his message about the state of the country and his victory. Hitting the campaign trail early, even to face off against his own party, might be something he would welcome.

There is no doubt plenty of Republicans are at least considering challenging Trump, perhaps due to his high disapproval ratings. On Tuesday, a Rasmussen Reports tracking poll showed that 50 percent of likely voters disapprove of the president.

That could present an opportunity for a number of key Republicans. Perhaps the most obvious would be Sen. Marco Rubio of Florida, who was subjected to "Little Marco" taunts by Trump on the campaign trail. Rubio, who will be 49 years old at the time of the next presidential election, has shown a willingness to defy the White House when he expressed reservations about Trump's choice for secretary of state, Rex Tillerson. He also openly criticized the decision to ban refugees from Muslim-majority countries.

By 2020, Rubio could be more of a factor in the presidential race if he manages to appeal to independents by challenging Trump on controversial legislation. It also helps that Rubio is from a crucial swing state with 29 electoral votes.

Another Republican candidate who may throw his hat into the race is Sen. Ted Cruz of Texas. Much will be learned about Cruz's White House aspirations when he runs for re-election in 2018. A strong showing along with dipping approval ratings for Trump may give Cruz enough reason to make another run. Cruz didn't exactly give Trump a ringing endorsement at the Republican National Convention, which is understandable given how Trump repeatedly disparaged both Cruz and his family. 

A Politico report by Burgess Everett pointed out that the differences between Trump and Cruz could lead to the 46-year-old making another run. "Though he won’t say it, Cruz’s skeptical stance could position him to run in 2020 if Trump’s first term is seen as a failure, particularly among staunch conservatives who preferred Cruz to begin with," Everett wrote. 

Other potential candidates may include Gov. John Kasich of Ohio, Sen. Rand Paul of Kentucky and businesswoman Carly Fiorina. All of the aforementioned candidates ran for president in 2016. Meanwhile, newcomers like Sen. Tim Scott of South Carolina may also weigh a presidential run.