Republican presidential candidate Mitt Romney introduced Wisconsin congressman Paul Ryan as his running mate Saturday, choosing a politician who is viewed as a bridge between the Republican establishment that backs Romney and the small-tax, anti-big government tea party movement that mistrusts him.

Mr Ryan is the architect of a deeply conservative and intensely controversial long-term budget plan to cut trillions in federal spending, and his conservative credentials are highly regarded by fellow Republican House members.

Many polls during the primaries of winter and spring found that Mr Romney's own were suspect among the party's core, tea-party supporters.

With his pick, Mr Romney sought to boost his own image among the conservatives, repair an image damaged by negative Democratic advertising and shift the trajectory of a campaign that has seen him lose ground to President Barack Obama.

Mr Ryan came out swinging, blaming the president for the country's sluggish economy.

"Regrettably, President Obama has become part of the problem, and Mitt Romney is the solution" to an economy that has yet to make a strong recovery from the worst recession in decades, he said.

Mr Romney exulted in his choice, telling cheering supporters gathered before them that "I did not make a mistake with this guy".

In the campaign to come, Republicans will present economic solutions "that are bold, specific and achievable," Mr Romney said. "We offer our commitment to create 12 million new jobs and bring better take home pay to middle class families."

The ticket-mates made their first joint appearance at a naval museum, the initial stop of a bus tour through four battleground states in as many days.

Having Mr Ryan on the ticket also could help Mr Romney become more competitive in Wisconsin, a state Mr Obama won handily four years ago in the state-by-state race for the presidency, but that could be much tighter this November.

At same time, Democrats have been eager for Mr Romney to choose Mr Ryan, so they could pin what they see as an unpopular budget plan that calls for deep cuts in government spending and overhaul entitlement programmes to the Romney campaign.

"The architect of the radical Republican House budget, Ryan, like Romney, proposed an additional $250,000 tax cut for millionaires, and deep cuts in education," Jim Messina, the president's campaign manager, said in a written statement.

At 42, Mr Ryan is a generation younger than the 65-year-old Mr Romney.

A seventh-term congressman, Mr Ryan is chairman of the House Budget Committee, and primary author of conservative tax and spending blueprints that the tea party-infused Republican majority approved over vociferous Democratic opposition in 2011 and again in 2012.

It envisions transforming Medicare - the national health insurance programme for Americans age 65 and older - into a programme in which future senior citizens would receive government cheques that they could use to purchase health insurance. Under the current programme, the government directly pays doctors, hospitals and other health care providers.

Mr Ryan and other supporters say the change is needed to prevent the programme from financial calamity. Critics argue it would impose ever-increasing costs on seniors.

Other elements of the budget plan would cut projected spending for Medicaid, the government plan that provides health care for the poor, as well as food stamps, student loans and other social programs that Mr Obama and Democrats have pledged to defend.

In all, it projected spending cuts of $5.3 trillion over a decade, and cut future projected deficits substantially.

It also envisions a far reaching overhaul of the tax code of the sort Mr Romney has promised.

In turning to Mr Ryan, Mr Romney bypassed other potential running mates without the Wisconsin lawmaker's following among rank-and-file conservatives, including Ohio Senator Rob Portman and former Minnesota Governor Tim Pawlenty.

Republican officials said Mr Romney had spoken with both men.

Mr Romney and Mr Ryan appeared unusually comfortable with each other when they campaigned together earlier in the year. The former governor eagerly shared the microphone with the younger man and they shared hamburgers at a fast food restaurant.

In making an endorsement before his state's primary last spring, Mr Ryan said, "I picked who I think is going to be the next president of the United States - I picked Mitt Romney. ... The moment is here. The country can be saved. It is not too late to get America back on the right track. ... It is not too late to save the American idea."

In recent days, conservative pundits have been urging Mr Romney to choose Mr Ryan in large part because of his authorship of the House-backed budget plan that seeks to curb overall spending on the benefit programmes.

Republican National Committee finance chairman Ron Weiser of Michigan, said last night that Mr Ryan's selection would help Mr Romney win Wisconsin and its 10 electoral votes in the fall. The state typically supports Democrats in presidential contests, and Obama won it handily four years ago.

Mr Ryan has worked in Washington for much of his adult life, a contrast to Mr Romney, who frequently emphasises his experience in business.

The congressman worked as an aide in Congress, and also was a speechwriter for Jack Kemp, who years earlier had been one of the driving forces behind across-the-board tax cuts that were at the heart of Ronald Reagan's winning presidential campaign in 1980.

Mr Ryan is also well-known for his fiendish physical fitness workouts.

His congressional district in southeast Wisconsin has something of a bipartisan voting record. Mr Obama took 54% of the vote there in 2008, while the congressman received 64% in winning re-election.

Most of Mr Romney's staff learned of the planned announcement during a conference call last night about an hour before the campaign issued a statement.

The identity of Romney's pick was not disclosed during the call. The campaign had promised that first news of the selection would be delivered via a phone app.