The senator of the US state of Texas, Ted Cruz, has announced he will run for president in the 2016 elections, becoming the first major figure from either party to do so.
Cruz, an unabashed conservative who frequently clashes with leaders of his Republican Party, made the announcement on Twitter.
"I'm running for President and I hope to earn your support!" Cruz, 44, said on his official Twitter page.
The message was accompanied by a 30-second video in which Cruz said: "I believe in America and her people, and I believe we can stand up and restore our promise."
American scenes from southwestern landscapes to the Golden Gate Bridge were also shown.
"It's going to take a new generation of courageous conservatives to help make America great again," Cruz said, adding that he was ready "to lead the fight".
Saying Twitter is an unusual forum for presidential candidates to announce such plans, the Associated Press news agency nonetheless reported that Cruz's early announcement of candidacy could give him a head-start in what is likely to be a crowded field of Republican White House contenders.
An unnamed aide cited by AP said Cruz, a favourite of the Tea Party, a conservative pressure group allied with the Republican party, was expected to speak on Monday at Liberty University, an evangelical Christian school in Virginia.
Cruz has built a reputation as an unyielding advocate for conservative principles in his two years in the Senate.
As a presidential candidate, he is likely to portray himself as the most reliable proponent of small government, reported the AP.
"We need to look to people who walk the walk and don't just talk the talk," he told the Conservative Political Action Conference in February.
More than a dozen potential presidential candidates are already courting donors and voters in states like Iowa and New Hampshire that vote early in next year's primary season.
But none are expected to enter the race formally until April at the earliest, in order to maximise their advantage under campaign finance laws.
Would-be contenders from the Republican party that have yet to declare their candidacy include Jeb Bush, Florida governor, and Marco Rubio, Florida senator.
The Democratic field is shaping up to be far leaner, with only Hillary Clinton, the former secretary of state, said to be eyeing the presidency and is viewed as the front-runner in her party.
Cruz is a Canadian-born son of a Cuban immigrant and would be the first Hispanic in the White House if he won the November 2016 polls.