His audience: Several hundred field organizers for Americans for Prosperity, the Tea Party-inspired group founded by the billionaire conservative philanthropists Charles and David Koch.
      
When Mr. Romney announced that Mr. Ryan would be his running mate, his campaign emphasized the congressman’s detailed knowledge of the federal budget and his chemistry with Mr. Romney.
Less well-known are Mr. Ryan’s close ties to the donors and activists who have channeled Tea Party anger into a $400 million political machine, financed by a network of conservative and libertarian donors that now rivals, and occasionally challenges, the Republican establishment behind Mr. Romney.
      
Mr. Ryan is one of a very few elected officials who have attended the Kochs’ biannual conferences, where wealthy donors sit in on seminars on runaway government spending and the myths of climate change.
      
He is on first-name terms with prominent libertarians in the financial world, including hedge fund billionaires like Cliff Asness and Paul Singer, and spent his formative years immersed in the Republican Party’s supply-side wing, working for lawmakers and conservative policy advocates like Jack Kemp.
      
He has appeared for years at rallies, town hall meetings, and donor briefings for groups like the Club for Growth, which spends millions to defeat Republicans deemed squishy on taxes and spending, and Americans for Prosperity, a grass-roots group focused on economic and budget issues that is now trying to channel Tea Party energy into a permanent electoral force. Its fourth chapter was founded in Mr. Ryan’s home state, Wisconsin.
      
Now Mr. Ryan could provide Mr. Romney with a critical political and intellectual bridge to the rising conservative counterestablishment represented by the Kochs and their allies, who are planning to spend hundreds of millions of dollars and deploy thousands of volunteers to defeat Mr. Obama.
Should Mr. Romney and Mr. Ryan win in November, a constituency that has for years fulminated against the failure of Republicans to live up to their own principles could soon have a close — and powerful — friend in the White House.
      
“There’s three guys that we courted for president: Paul Ryan, Mitch Daniels, and Mike Pence,” said Matt Kibbe, the president of FreedomWorks, a national advocacy group closely allied with the Tea Party, who worked alongside Mr. Ryan when both were staff aides on the House Budget Committee. “Up until yesterday, there was a 100 percent commitment to fire Obama. There was not a lot of enthusiasm about Romney.” Mr. Daniels is the governor of Indiana, and Mr. Pence is a congressman from Indiana.
      
Mr. Kibbe added, “From a Tea Party perspective, the overwhelming response on all of our networks has been extremely positive.”
      
Mr. Ryan’s ties to that world began with a job at Empower America, a group founded by Mr. Kemp that ran “candidate schools” for aspiring conservatives and advocated for a flat tax and lower spending. As a rank-and-file congressman during the presidency of George W. Bush, Mr. Ryan advocated for the privatization of Social Security, helping push the idea toward the Republican mainstream and cementing his reputation as a conservative intellectual.
      
Privately, Mr. Ryan would later say, he was also stewing over what he and other conservatives viewed as the Bush administration’s fiscal profligacy and ideological drift, including the addition of a drug benefit to Medicare and, later, a bank bailout plan, the Troubled Asset Relief Program. (Mr. Ryan voted for both.)
      
That dissatisfaction was shared by the Kochs, who in the middle of the last decade began organizing conferences of like-minded donors and founded Americans for Prosperity.
      
Mr. Ryan, who became House budget chairman in 2006, began attending and speaking at Americans for Prosperity events. In 2008, the Wisconsin chapter gave Mr. Ryan its annual “Defender of the American Dream” award. Mr. Ryan also began attending the Kochs’ annual donor seminars. Last spring, Mr. Ryan was a speaker at a “Hands Off My Health Care” rally organized by Tea Party leaders outside the Capitol, drawing enthusiastic applause.
      
In Congress, he emerged as a skeptic of mainstream climate change theory — opposition to which has been a top priority of Koch-affiliated activists and research groups — and a reliable vote against energy efficiency standards, including a House vote to prohibit the Environmental Protection Agency from regulating greenhouse gases.
      
The relationship helped Mr. Ryan’s campaign coffers as well as his career: the Koch Industries PAC has donated more than $100,000 to Mr. Ryan’s campaigns and his leadership PAC, more than has any other corporate PAC, according to a New York Times analysis of campaign records.
      
Mr. Ryan has also developed relationships with other people in the Koch orbit, like Mr. Asness, a libertarian-minded financier known for his open letters blasting Mr. Obama, and Kenneth Griffin, a Chicago hedge-fund executive: wealthy donors whose taste for number-crunching and policy minutiae match Mr. Ryan’s own.
      
Mr. Griffin and his wife, Anne, introduced Mr. Ryan to Chicago’s deep-pocketed Republican donor circle — he has raised more money there this campaign than any other city — and promoted his budget proposals, including arranging a speech last year at the Economic Club of Chicago.
      
But it was Mr. Ryan’s aggressive promotion of his budget plan that has cemented his place the counterestablishment’s rising star. Mr. Ryan’s plan, viewed warily in its early form by other Republican leaders on the Hill, became an organizing document for the Tea Party’s Beltway wing, particularly the dozens of Tea Party-inspired freshman lawmakers who arrived on Capitol Hill after the 2010 elections. Many of them came to rely on Mr. Ryan for counsel on whether to accept budget compromises with Mr. Obama.
      
Outside political groups and research organizations praised Mr. Ryan’s plan, one of the few comprehensive conservative budget proposals detailed enough to be scored by the Congressional Budget Office, as rigorous and credible.
      
“Paul was one of the first guys that we looked at and said, ‘Hey, that young guy could be the guy,’ ” said Tim Phillips, Americans for Prosperity’s president. “And when he put out the budget and defended it, that’s when they said, ‘He could go all the way.’ ”
      
Officials with several outside groups that had been skeptical of Mr. Romney in the past said that the selection of Mr. Ryan had assuaged some of their doubts.
      
More important, they said, Mr. Ryan would fire up their grass-roots members, some of whom had doubted Mr. Romney’s commitment to cutting the size of government. Last week, before the announcement, Americans for Prosperity announced that it had begun its largest ever ad campaign against Mr. Obama, a $25 million broadside in 11 battleground states.
      
And on Monday, Romney officials said that the campaign had raised millions of dollars in the wake of Mr. Ryan’s selection, not only from grass-roots small donors, but from the many big donors who rank among his fans.
Griff Palmer contributed reporting.