"This challenges the whole argument that Democrats have made that the environment is so bad that the stability in the [political] system would be shaken," said Ken Mehlman, chairman of the Republican National Committee.
Rothenberg and other analysts said Busby's showing should be most sobering to Democrats arguing that this year's atmosphere of discontent would allow the party to significantly "expand the battlefield" into traditionally Republican-leaning districts.
"It is certainly a cautionary note about that," agreed a Democratic strategist who works closely with the party's congressional leadership and requested anonymity when discussing the private reaction to Tuesday's vote.
Some Democrats remain confident they can win the House without winning many -- or perhaps any -- of the districts that lean as much to the GOP as Cunningham's former seat. For that to happen, Democrats need to maximize their gains in Democratic-leaning and swing districts held by Republicans across the Northeast and industrial Midwest.
The Bilbray-Busby race "tells us Republicans aren't voting Democratic; we know that," Rothenberg said. "But what about Democrats who have been voting for Republican candidates? It doesn't tell us a lot about that."
The race did offer some guidance about the political dynamics of the immigration debate.
Echoing the position of the House GOP leadership, Bilbray called for intensified border security and a crackdown on illegal immigrants. He also seemed to have benefited from Busby's gaffe on the subject.
His win is likely to reinforce the reluctance among House Republicans to accept Bush's broader approach to immigration policy, which includes a plan that would potentially legalize millions of undocumented workers.
In Montana, Burns is one of the most politically vulnerable Senate Republicans because of contributions he received from Abramoff or the lobbyist's associates.
Democrat John Morrison, the state auditor, for months had been viewed as his likely opponent, in part because he had raised significantly more money than his chief rival for the nomination, state Sen. Jon Tester.
But Morrison lost his edge after revelations of an extramarital affair. That gave Tester an opening to argue that he was the candidate best able to run against Burns on the ethics issue.
In what had been expected to be a close race, Tester trounced Morrison, 61% to 35%.
