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

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

Mr. Romney made his announcement to supporters via a phone app. “Mitt’s Choice for VP is Paul Ryan,” it said and implored backers to spread the word.

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

In a written statement issued a short while later, Mr. Romney’s campaign said that Mr. Ryan had worked in Congress to “eliminate the federal deficit, reform the tax code and preserve entitlements for future generations”.

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

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.

Republican National Committee finance chairman Ron Weiser of Michigan, said Friday 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 Mr. Obama won it comfortably four years ago.

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

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