Former Senator Rick Santorum and former New York Governor George Pataki have formally sought the Republican nomination for the November 2016 US presidential election while real estate mogul and TV personality Donald Trump said he will make an announcement of his own candidacy next month.

Santorum, a 57-year-old devout Catholic, announced his second presidential bid on Wednesday at his home in Pennsylvania, according to the Catholic News Agency (CNA).

"I am proud to stand here among you and for you, the American workers who have sacrificed so much, to announce that I am running for president of the United States," said Santorum.

He is the third Republican Catholic to run for president besides Jeb Bush and Marco Rubio.

Santorum said in 2011 that he is "proud of being Catholic."

"I'm proud of the teachings of the Church," he told CNA in 2011. He placed second to Mitt Romney in the 2012 Republican presidential derby.

In his speech, Santorum promised to restore the economic power of middle-class American workers, according to Reuters.

"Today is the day we are going to begin to fight back," he said. "As middle America's hollowing out we can't sit idly by. Working families don't need another president tied to big government or big money."

He also said he will eliminate the Internal Revenue Service, and crack down on illegal immigration that he claimed robbed jobs from Americans.

On the other hand, Pataki announced his presidential bid in Exeter, New Hampshire, immediately launching a blistering attack on Democratic presidential nominee Hillary Clinton.

"She (Clinton) speaks for the middle class? They are the party of privilege; we are the party of the middle class," said Pataki, according to Reuters.

When he was governor of New York, Pataki supported abortion rights and signed gun control legislation.

He said on Thursday that his "conservative policies" slashed the number of people on welfare.

He acknowledged that it would be a difficult campaign for him compared to other Republican presidential nominees who are more popular.

"It will be a very stiff climb up a very steep mountain, but that hasn't stopped me in the past," Pataki told the New York Post this week.

On the Islamic State, Pataki said he supports deploying limited ground troops against the terrorist organization.

"And yes, if necessary, American forces will be used to actually defeat and destroy ISIS so they can pose no threat to us here," he said.

He also called for a simpler US tax code and promised to repeal Obamacare.

Trump said he will make a major announcement on June 15 in New York regarding his presidential bid, according to TV station WMUR on Thursday.

Trump, 68, previously told Reuters that he had formed an exploratory committee for his potential presidential run and hired staff in Iowa, New Hampshire and South Carolina.