TAMPA, Florida (Reuters) - Paul Ryan promised on Wednesday that he and Republican presidential contender Mitt Romney would make the tough choices needed to lead a U.S. economic turnaround that generates jobs and cuts government spending and debt.

Ryan accepted his nomination as Romney's running mate at the Republican convention in Tampa, Florida, drawing repeated roars from delegates as he vowed to challenge President Barack Obama's economic policies and confront Democrats on changes to the popular Medicare health program for seniors.

A fiscal conservative and budget expert, Ryan said the White House race would offer "the clearest possible choice" in the November 6 election about possible remedies for a sluggish economy and high unemployment.

"Before the math and the momentum overwhelm us all, we are going to solve this nation's economic problems," Ryan said in a speech that introduced the little-known Wisconsin congressman to voters.

"We will not duck the tough issues - we will lead. We will not spend four years blaming others - we will take responsibility," he said. "So here's the question: Without a change in leadership, why would the next four years be any different from the last four years?"

Ryan pointed out his elderly mother, Betty, in the audience when talking about Medicare and drew laughs when he mocked Romney's old-fashioned choices in music.

But the Obama campaign criticized Ryan for misleading voters on Medicare, a deficit reduction plan in Congress and even a factory closure in his hometown of Janesville, Wisconsin.

"If Paul Ryan was Pinocchio his nose would be back in Janesville about now," Obama campaign spokeswoman Jen Psaki said on Twitter.

Romney's selection of Ryan, chairman of the House of Representatives Budget Committee, has energized party conservatives who have doubted Romney at times and has put Ryan's proposed changes to Medicare at the center of the campaign debate.

Ryan's budget plan would rein in government spending and shift some Medicare participants into private insurance plans purchased with the help of government subsidies, a proposal that Democrats charge would put future benefits for seniors at risk.

On the second full day of the convention, Republicans also criticized Obama's foreign policy and featured a parade of women and Hispanic speakers trying to broaden Romney's appeal.

Republicans hope to strike a balance at the convention between sharp indictments of Obama's leadership and a broader introduction of Romney's plans for the economy and the softer side of a candidate who has had trouble connecting with voters.

It seems to be working for Romney so far. A Reuters/Ipsos online poll on Wednesday showed him deadlocked with Obama among likely voters at 43 percent each - an improvement for Romney from Obama's two-point lead on Tuesday and four-point lead on Monday.

ROMNEY'S BIG NIGHT

Ryan's speech set up Romney's big night on Thursday, when he accepts the presidential nomination in a major speech. The former private equity executive must make Americans feel more comfortable with him while laying out a path to economic and an end to high unemployment.

Then the campaign season moves into the two-month final stretch after Democrats meet next week in Charlotte, North Carolina, for their own convention.

The strengths of budget hawk Ryan and ex-businessman Romney are on the economy, but the Republicans also brought out foreign policy heavyweights at the convention.

Condoleezza Rice, who served as secretary of state under former Republican President George W. Bush, said Romney and Ryan were ready to lead the United States back to a role of international leadership.

"Mitt Romney and Paul Ryan will rebuild us at home and inspire us to lead abroad. They will provide an answer to the question, "Where does America stand?" she said.

Arizona Senator John McCain accused Obama of allowing the decline of U.S. influence.

At an American Legion gathering in Indianapolis, Romney tried to counter Democratic criticism of his inexperience abroad and accused Obama of failing to stand by America's allies.

"In dealings with other nations, he has given trust where it's not earned, insult where it's not deserved and apology where it's not due," he said

Foreign policy and military matters are points of vulnerability for Romney. A trip abroad last month aimed at burnishing his credentials was plagued by gaffes and stumbles.

But U.S. voters are firmly focused on the economy at home.

Ryan promised Republicans they could win the Medicare debate with Democrats.

"Medicare is a promise, and we will honor it. A Romney-Ryan administration will protect and strengthen Medicare, for my mom's generation, for my generation, and for my kids and yours," he said to cheers.

"So our opponents can consider themselves on notice. In this election, on this issue, the usual posturing on the left isn't going to work," he said.

Republican delegate Ronda Vuillemont-Smith of Oklahoma said the selection of Ryan would help focus the ticket on spending cuts and reducing the size of government.

"I honestly feel that Ryan brought backbone to the ticket," she said.