Ryan is expected to accept the nomination as Mitt Romney's vice presidential running mate at the Republican National Convention on Wednesday.
A question mark continues to hang over the convention as Hurricane Isaac slams into the US Gulf Coast. The storm missed Tampa but has cast something of a pall over the convention because delegates are loathe to seem overly joyful while Americans elsewhere suffer.
Heading the list of speakers on Wednesday is Ryan, a conservative budget hawk from Wisconsin.
Careful not to emulate predecessor Sarah Palin, who fell from grace quickly after bursting onto the 2008 campaign as John McCain's running mate, Ryan has made a cautious start to the presidential race.
It is still unclear whether he will help Romney draw support from undecided voters who may be the critical factor in the November 6 presidential election. Polls show a mixed picture.
"Tonight, the American people – millions who may not know a lot about Paul Ryan, other than the headlines that they've read – are going to get to know Paul Ryan the way many of us know him: as a serious policy thinker," Republican US Senator Marco Rubio told ABC's Good Morning America on Wednesday.
Rubio, who represents swing state Florida and its many seniors, defended Ryan's controversial budget plan to cut government spending deeply and overhaul the government-run Medicare health insurance program for older Americans.
There has been no doubt that Ryan has energised conservatives in a way Romney was unable to do during the long months of the Republican primary battle, when he faced a number of conservative challengers.
The boyish 42-year-old Ryan, a fitness fanatic, has shown himself to be an affable asset to Romney so far.
He has helped generate large crowds for Romney when the pair has campaigned together, and some conservatives who were not that excited about the former governor of Massachusetts are now ready to work hard for him with Ryan on the ticket.
Ryan also helps put in play Wisconsin, a Midwestern state that has not voted for a Republican presidential candidate since Ronald Reagan in 1984. A Romney victory in Wisconsin could alter the electoral map in a way that could hurt President Barack Obama's hopes for re-election.
Ryan's place in prime time on Wednesday offers him the chance to introduce himself to millions of Americans who are just starting to tune in to a presidential race that is too close to call with 70 days left until the voting.
While Ryan, chairperson of the House of Representatives Budget Committee, is well known in Washington, he is little known elsewhere.
Democrats are portraying Ryan as an extreme conservative ideologue whose budget proposal in the House would "end Medicare as we know it" and are using his budget plan against him in states like Florida, with its large population of retirees, and in Virginia, where thousands of government employees populate the suburbs adjoining Washington, DC.
Romney can ill afford to lose either of those two states.
A Reuters/Ipsos poll on Tuesday found that half of Americans approve of Ryan and the other half disapprove of him.
Speaking before Ryan will be Condoleezza Rice, who served as secretary of state under former Republican President George W Bush. She told CBS' This Morning that she would press delegates about "the importance of rebuilding America at home so that we have a firm foundation from which to lead abroad".
Rice told CBS she would not accept a position in Romney's administration if he won the November 6 election.
Other speakers at the Republican convention have sought to put a human face on the often robotic Romney and enhance his likability. On Tuesday, there was no better advocate for him than his wife, Ann Romney.
She admitted to reporters she had never used a prompting device to read a speech, but during the actual delivery she seemed at ease as she painted a personal portrait of Romney, who Democrats denounce as an out-of-touch wealthy elitist.
Mrs Romney spoke of the early years of their marriage when the high school sweethearts dined on cheap meals of tuna and pasta, saying her husband was "not handed success" as Romney's opponents charge.
Rubio, who is scheduled to introduce Romney ahead of the Republican presidential nominee's speech on Thursday, said such soft details about Romney are unlikely to come from Ryan's remarks.
"If I know Paul Ryan, we're going to get a policies speech that's also inspiring," he told CBS. – Reuters