Paul Ryan accepted the Republican nomination as Mitt Romney's vice presidential running mate last night, saying the moment for President Barack Obama's Democrats "came and went".
Mr Ryan's nationally-televised speech on the second day of the storm-shortened Republican National Convention in Tampa, Florida, was a debut of sorts for the 42-year-old congressman from the Midwestern state of Wisconsin.
Though a leader on budget policy in Congress, Mr Ryan was not well-known outside Washington when tapped by Mr Romney this month.
The selection of Mr Ryan, author of a plan to reduce the government deficit, excited Republicans sceptical of Mr Romney's commitment to conservative principles.
But Democrats pounced, saying Mr Romney, the former governor of Massachusetts, was now clearly wedded to Mr Ryan's proposals to cut spending by revamping health care programmes for the elderly and poor.
To the cheers of the Republican faithful, Mr Ryan said Democrats "have run out of ideas. Their moment came and went. Fear and division is all they've got left".
He said Mr Romney would not duck the difficult decisions needed to repair the economy.
"After four years of getting the runaround, America needs a turnaround, and the man for the job is Governor Mitt Romney," he said.
The speech came at a gathering struggling for attention as Tropical Storm Isaac cast a pall from the nearby northern coast of the Gulf of Mexico. The storm had threatened Florida earlier in the week and prompted Republicans to postpone Monday's start of the convention.
Mr Romney and Mr Ryan were formally nominated in roll call votes on Tuesday. Mr Romney accepts his party's nomination in a nationally-televised speech tonight, US time, the third and final full day of the convention.
But he may find Mr Ryan a tough act to follow. His speech was part attack on Mr Obama, part spirited testimonial to Mr Romney, all leavened by a loving tribute to Mr Ryan's own mother, seated across the hall in a VIP box.
"To this day, my mom is a role model," he said while she beamed and exchanged smiles with one of his children and delegates cheered.
Mr Ryan's youthful energy and down-to-earth appeal stands in contrast to the stiffer, more aristocratic Mr Romney, 65. He drew laughs in his speech joking about Mr Romney's musical tastes.
Still, so far Mr Ryan has not changed the dynamics of the presidential race. Polls continue to show Mr Romney and Mr Obama in a statistical tie ahead of the November vote.
The economy is the biggest issue in the race. While voters have more confidence in Mr Romney on economic matters, they like Mr Obama better on a personal level.
A poll by the Pew Research Centre and The Washington Post found Americans deeply divided about Mr Ryan.
Traditionally, vice-presidential picks have little effect on US presidential elections, though John McCain's selection of then-Alaska governor Sarah Palin jolted the race four years ago.
Her electrifying speech was the highlight of the 2008 convention, but her poor performance in subsequent interviews left the widespread impression she was unprepared for the vice presidency.
Mr McCain spoke before Mr Ryan. Without mentioning Mr Obama by name he accused the president of failing to lead on defence spending and grave international issues.
"Sadly, for the lonely voices of dissent in Syria and Iran and elsewhere who feel forgotten in their darkness … our president is not being true to our values," he said.
Condoleezza Rice, secretary of state under President George Bush also did not mention Mr Obama by name, but implicitly criticised his leadership in foreign affairs: "We cannot be reluctant to lead and you cannot lead from behind," she said.
She said Mr Romney and Mr Ryan "will provide an answer to the question, 'Where does America stand?'."
The comments of Ms Rice and Mr McCain marked some of the few moments in which international affairs have received attention at the convention. Neither Mr Romney nor Mr Ryan has extensive international experience.
Opinion polls show Mr Obama getting high marks on national security after ending the war in Iraq, drawing down the conflict in Afghanistan and ordering the killing of terrorist leader Osama bin Laden.
But Mr Romney, speaking to a veterans group in Indianapolis, said Mr Obama "has allowed our leadership to diminish".
"In dealings with other nations, he has given trust where it is not earned, insult where it is not deserved and apology where it's not due," he said.
Mr Obama, campaigning before a university crowd in Virginia, declared himself unimpressed with the Republican convention.
"You can listen very carefully, very hard, and you won't hear them offer a clear serious path forward," he said.