Remember "hockey mom" Sarah Palin in '08? On Wednesday it's the turn of youthful Wisconsin congressman Paul Ryan to wow the Republican convention as he accepts the party's vice presidential nomination.

At 42, Ryan is two years younger than the former Alaska governor was when she was thrust into prime-time as Senator John McCain's running-mate, later showing her inexperience as she came unstuck on policy on the losing presidential ticket.

Experience should not be a problem for Ryan, a self-confessed number cruncher and policy wonk who has six-plus terms under his belt as a congressman and whose recent budget plan is the Republican blueprint to fix the troubled economy.

But nerves could still play a part as he prepares to make the biggest speech of his life just 10 weeks out from an election that will decide if President Barack Obama is re-elected or Mitt Romney gets the keys to the White House.

The Obama campaign upped the pressure on the photogenic rising star of the Republican Party in the hours leading up to his big moment, releasing a new web video that asked why Americans are so nervous about Paul Ryan.

"Is it his commitment to the failed top-down economics of the past with tax cuts for the wealthiest?" it said, accusing Ryan's budget plan of seeking to end the Medicare system for the elderly by introducing a new voucher program.

After would-be first lady Ann Romney took to the podium on Tuesday night, humanizing her at-times awkward husband and making a targeted pitch to crucial women voters, the Obama team also used the video to strike back.

"Perhaps it's Paul Ryan's views towards women that makes Americans nervous: sponsoring anti-abortion legislation that used the term forcible rape," it said, as polls show Romney trailing Obama with young females.

In Wednesday night's speech to a packed convention center in Tampa, Florida, don't expect to hear much though from Ryan on divisive social issues.

Instead, he will focus on introducing himself to American voters, lashing Obama policies that have mired the flagging US economy, and holding up Romney, a 65-year-old former Massachusetts governor, as the man who can turn things around.

Ryan speech writer Matthew Scully also wrote Palin's barnstorming speech in 2008, when she tore strips from Obama and went after the liberal news media, portraying herself as "just your average hockey mom."

The Romney campaign has been deliberative throughout the VP nominating process, trying to learn from McCain's mistakes four years ago when the Palin choice and the mixed messages the pair put out ultimately led to their downfall.

Romney and Ryan have exhibited an easy rapport in their few weeks of campaigning since the VP pick was announced and there is at least one striking similarity between the two: both are sons of successful fathers in the Midwest.

During the roll-out earlier this month, Romney praised his running mate as an "intellectual leader in our party," someone who earned respect from Republicans and Democrats alike for having "made friends on both sides of the aisle."

Ryan's speech, at 0200 GMT Thursday, follows a well-received effort by Romney's wife on Tuesday's storm-delayed opening night of the convention to sell the couple's family and sweetheart love story to undecided US voters.

Romney, a multi-millionaire businessman who the Obama campaign has attacked for his tax secrecy, will formally take up the nomination with his all-important acceptance speech on Thursday, the climax of three days of rousing convention addresses by party grandees and rising stars.

Romney lies neck-and-neck with Obama in national polls ahead of a November election that should be the challenger's for the taking, given the sour economy and stubbornly high unemployment.