US Republican presidential candidate and Wisconsin Governor Scott Walker formally announces his campaign for the 2016 Republican presidential nomination during a kickoff rally in Waukesha, Wisconsin, July 13, 2015. [Photo/Agencies]

WASHINGTON - Wisconsin State Governor Scott Walker on Monday announced his decision to enter the 2016 US presidential election, becoming the 15th Republican candidate to join the race.

Walker delivered his campaign speech in Waukesha, Wisconsin, Monday afternoon, making such promises as to repeal the Affordable Care Act, commonly known as ObamaCare. He is said to have appeal among GOP conservatives and to be a rival for the Republican front- runner Jeb Bush.

Walker is also one of critics who have suggested the US is offering too many concessions to Tehran in the negotiations for a deal to terminate the decade-old standoff over Iran's nuclear program.

Democrats hit Walker hard before his campaign speech. Earlier in the day, Democratic presidential candidate Hillary Clinton accused Walker of having made his name by "stomping on workers' rights."

Walker's campaign launch was attended by about 5,000 supporters. The 47-year-old, who is at his second term as governor, will be visiting the state of Nevada Tuesday, South Carolina Wednesday, New Hampshire Thursday and Iowa Friday through Sunday. He will hold at least 10 public events in Iowa alone, according to his campaign team.