US president George Bush has told a big meeting of the Republican party that he's backing John McCain to be the next leader of America.
Appearing on a big screen he said Senator McCain was "a great American and the next president".
Mr Bush can't put himself up for the job again, because the rules say he's been in the job too long to run again.
To become president, Mr McCain has to beat Barack Obama in the November presidential elections.
President Bush said Mr McCain would be a president ready to make tough decisions "in a dangerous world".
John McCain must beat Barack Obama
He said: "John McCain's life has prepared him to make those choices. He is ready to lead this nation."
Mr McCain is due to be officially nominated as the Republican party candidate on Thursday.
His opponent, Barack Obama, accepted the Democratic party's nomination in front of cheering crowds of 80,000 in a football stadium in Denver last week.