Romney's conservative card
The selection of Paul Ryan as the Republican presidential hopeful Mitt Romney's vice-presidential running mate is a hugely significant moment.
It clearly lays out that the US presidential election is not simply going to be about the economy - we all knew that - this is going to be an ideological battle about the future shape of government in America.
The 42-year-old congressman from Wisconsin was always one of the names being considered by Romney for a place on the ticket, but in the last few days the calls for his inclusion have grown stronger, with even the influential Wall Street Journal weighing in.
In an editorial it wrote: "The case for Mr Ryan is that he best exemplifies the nature and the stakes of this election."
The Romney team insists it made the decision around a week before that appeared, the candidate impressed and comfortable with the man with whom he'll share the campaign trail.
Picking Ryan will ignite the Republican base and the wing of the party which has never been convinced that Mitt Romney was conservative enough.
Tea Party darling
A father of three, Ryan is a darling of the Tea Party, a socially conservative Catholic who opposes abortion under most circumstances and same sex marriage, but is less confrontational than others with the same views.
He is the current chairman of the powerful House Budget Committee, an influential position which has seen him unpick the offerings from the White House while putting forward ideas of his own. Ideas which have been highly controversial, and that's where the ideological battle begins.
Ryan believes in much smaller government: large cuts in department budgets, huge cuts in entitlement programmes with no tax rises. That is politically at odds with Barack Obama's action of trying to stimulate economic growth by spending money on government building projects and trying to raise taxes on the top one per cent of earners.
During the presidency of George W Bush, Ryan was one of a number of conservative Republicans who suggested scrapping Social Security - money taken from people's pay packets to provide an old-age pension.
The idea didn't gain a lot of mainstream support, but it hasn't stopped Ryan coming forward with similar ideas.
He has recommended dramatically overhauling Medicaid, the government-financed health programme for retirees.
He would rather the government made direct payments to allow people to buy private insurance.
He also wanted to alter Medicaid, another healthcare programme which acts as a safety net for the poor who have no insurance to cover medical bills.
Ryan also talks about cutting other government spending although he hasn't given any clear indication about what would go.
He's promised to simplify the tax code - a familiar Republican refrain - but hasn't said how he would do that or what deductions that can currently be claimed would go. And he wants to cut taxes.
While Mitt Romney needs to make sure Republican voters actually going out and make a decision on November 6, he also needs independent swing voters to back him if he is to beat Barack Obama.
And that's what makes this choice controversial and potentially troublesome. Ryan's radical plans could have huge impact on older voters, who tend to turn out in large numbers in presidential election.
And that could be significant in important swing states like Florida and Nevada which have large numbers of elderly voters.
Democrats have been trying to tie Mitt Romney to Paul Ryan and his financial plans and government cuts for months. Mitt Romney has now just done that for them.
Ryan, though, is smart, savvy and very personable, so will be a huge asset on the campaign trail. He will be a formidable opponent for Joe Biden in the vice-presidential debate in the autumn.
And his selection changes the presidential campaign itself. There are many who wanted a substantive debate on important issues that really matter rather than a referendum on Barack Obama's first four years.
And now that is exactly what the American people have.