Washington, United States—US President Barack Obama has one wish for the Republicans trying to succeed him: that the odds are ever in their favor.

Obama poked fun at the ever-increasing "interesting bunch" of politicians vying for the Republican presidential nomination in the 2016 race, which now stands at 15.

"We’ve got some healthy competition in the Democratic Party, but I’ve lost count on how many Republicans are running for this job," the US leader said in a speech to students in the Midwestern state of Wisconsin.

"They will have enough for an actual Hunger Games," he joked, referring to the best-selling young adult novels, later transformed into successful blockbusters, about a bloody reality TV contest among children and teens in a dystopian future.

Wisconsin Governor Scott Walker, who welcomed Obama to his state on Thursday as he descended from Air Force One, signalled that he was joining the presidential fray, making him the 15th major Republican in the White House hunt.

The race features six current or former governors, five current or former senators, a former high-powered business executive and a neurosurgeon.

Former secretary of state Hillary Clinton is the Democratic frontrunner by a wide margin. Primary elections begin in January 2016, leading up to the presidential election in November that year.