CLEVELAND (AFP) - The Republican National Convention began the formal roll call vote Tuesday that will culminate in delegates nominating billionaire Donald Trump for president.

"The great state of Alabama... is proud to cast its votes: 1 vote for Marco Rubio, 13 for Ted Cruz, and 36 votes for the next president of the United States, Donald J. Trump!" US Senator Jeff Sessions said as he launched the process.

The real estate magnate was expected to cruise past the 1,237-delegate threshold needed to seal the deal on the first ballot and - if he follows tradition - Trump will officially accept the nomination on Thursday during his acceptance speech.

But it was not expected to be the smoothest of votes.

Minutes after the roll call began, Colorado's delegation chairman announced four votes for Trump, 31 for Senator Ted Cruz and two absentions, a result which drew murmurs from the assembled delegates.

Anti-Trump forces on the floor were holding out for a final miracle Tuesday, after seeking to convince delegates that their votes were not bound and that they could vote their conscience.

"Mainting order for the roll call is extremely important," House Speaker Paul Ryan, the convention chair, told the delegates before the vote.