For one of the U.S. Senate's true masters of international diplomacy and nuclear disarmament, there's no small irony in the fact that a home he sold in 1977 and the address that appears on his Indiana driver's license are now tripping him up more than any international Gordian Knot ever has.

U.S. Sen. Richard Lugar has largely skated through every re-election since he first won federal office in 1976. And even though he has consistently voted from a house he hasn't owned since he left for Washington in 1977, questions about his residency lay dormant until just a few weeks ago.

The story migrated after spending more than a year in Indiana's conservative blogosphere with the help of Democrats, tea partiers and a certified fraud examiner who investigated Lugar's residency late last year.

For decades, political strength, an attorney general's opinion and weak challenges from Democrats all kept the issue at bay. But Lugar's vulnerability this year has drawn a strong tea party challenge in the GOP primary as well as a solid Democratic challenger in Rep. Joe Donnelly, and the legality of his situation has done nothing to tamp the issue down.

Tony Long, vice chairman of the Indiana Election Commission, captured Lugar's dilemma shortly before voting to throw out a tea party challenge to Lugar's appearance on the May Republican ballot.

"I think he's clearly claimed himself to be a Hoosier. I'm sure he roots for IU when they play Kentucky," said Long, one of two Democrats on the commission. "Still, I'd feel a whole lot more comfortable if he had some residence here in the state."

Lugar stumbled through answers about his residency last week, telling reporters that he did not know the address on his driver's license shortly after saying that he renews his license himself. But he did hit on one key factor that has clouded his re-election effort, just as it has hung over his many Senate colleagues who have retired or suffered through grueling tea party challenges.

"Washington has changed over time," Lugar said shortly before speaking to a group named in his honor that trains Republican women to run for office and work on campaigns.

Lugar's supporters and campaign staff have called it ludicrous to attack a sitting senator for living where he works: in Washington. But in Indiana's current congressional delegation, Lugar sticks out. Most of Indiana's federal lawmakers own homes in Indiana and vote from that address, according to a search of the state's voter registration database and property tax records.

That Lugar doesn't has given more fodder to tea partiers already stewing over what they view as the senior senator's many political transgressions, including voting for President Barack Obama's Supreme Court nominee and blaming tea partiers for blowing Republicans' shot at winning back the U.S. Senate in 2010.

"I think what happened is the rank and file tea party felt betrayed. That's a really powerful emotion when you feel that you've been played," said Greg Wright, a certified fraud examiner who filed a complaint against Lugar in November.

Wright met with tea partiers opposing Lugar late last year and they brought up Lugar's residency. Until then, nobody had investigated the claims, keeping questions about his residency locked within a small confine of political insiders.

But it is now front and center with a major hand from Democrats, who have formed a truly strange alliance with tea partiers on the issue. Democratic Super PAC American Bridge has been running ads calling Lugar an excellent senator for Virginia, where he's lived since 1977. Indiana's Democratic Party, meanwhile, has held press conferences and pushed stories as Democrats try to build the storyline.

The last time Lugar ran for re-election, Democrats were more focused on a trio of tough Congressional races in 2006 than on fighting Lugar, who was at that point much stronger politically, said Democratic Party Chairman Dan Parker, who criticized Lugar for registering his Indianapolis farm at his U.S. Senate office in downtown Indianapolis.

What changed in six years? Democrats now have a top-tier Senate candidate in Donnelly, who opted to run for Senate after Republican lawmakers redrew his 2nd Congressional District last year.

"We have a strong candidate in Joe Donnelly," Parker said.

And with Lugar under fire from his own party, they also have a better platform for trotting out the issue.