By Nick Carey
(Reuters) - The last time Republican Senate veteran Richard Lugar stood for election, in 2006, he was so respected that the Democrats did not bother to run against him and he won his Indiana seat with 87 percent of the vote.
But now Lugar, 80, is faltering under a conservative challenge from within his own party in Tuesday's Indiana Republican primary. That is giving Democrats, struggling to keep control of the U.S. Senate in November's elections, a glimmer of hope of winning the U.S. Senate seat Lugar has held for 36 years.
Accused of worrying as much about India as Indiana, foreign policy expert Lugar is trailing his Tea Party movement opponent Richard Mourdock in polls in the state, by as much as 10 points in one survey.
Although traditionally Republican, Indiana has become more of a swing state in recent years. If small-government advocate Mourdock wins on Tuesday, Democratic candidate Joe Donnelly is seen as having a better chance of winning in the November general election than if the more moderate Lugar is the Republican candidate.
"It would be a much more competitive race with Mourdock in it," said Jennifer Duffy who tracks Senate races for the nonpartisan Cook Political Report.
The only poll so far pitting Donnelly, a U.S. congressman, against Mourdock, taken back in March, showed a tie, while six-term incumbent Lugar had a 20-point lead against the Democrat.
The Democrats are looking for any opening in their battle to keep control of the U.S. Senate, where they hold 53 of 100 seats. Twenty-three of the 33 seats up for grabs in November are Democrats' and around eight are in danger of falling to the Republicans, compared to three which might switch from Republican to Democratic.
If Obama is re-elected, Republicans need a net gain of four seats to control the Senate - to overcome the vice president's tie-breaking vote - and three if Republican Mitt Romney wins the White House.
That means Indiana could be crucial.
Obama won the state narrowly in 2008 thanks in part to a strong organization, but with the economy still weak and the presidential approval rating lukewarm it will be difficult for him to take Indiana in 2012. Donnelly cannot rely on hanging on to Obama's coat tails.
"Donnelly faces an uphill battle no matter who he's up against," said Brian Vargus, a political scientist at Indiana University-Purdue University Indianapolis. "It will be closer against Mourdock, but Mourdock will still have the advantage of Republican votes and money."
Marjorie Hershey, a political science professor at Indiana University, said Donnelly was "a more centrist Democrat of the kind that can get elected in Indiana. But this is a conservative state, so while Donnelly has a better shot against Mourdock he still faces long odds. This is a Republican race to lose."
One of the last of the Senate's "old guard," Lugar has a long track record of the kind bipartisanship that has gone out of fashion in Washington.
His expertise on foreign policy - from Pakistan to nuclear disarmament - has become a campaign burden as critics accuse him of forgetting the Midwestern values of Indiana, where he has not lived since 1977.
Conservatives fume over bipartisan votes by Lugar over the years, including approving President Barack Obama's Supreme Court nominees Sonia Sotomayor and Elena Kagan.
Lugar's response to his first primary challenge since his election in 1976 has been seen as outdated and lackluster.
"He (Lugar) hadn't had a real race in decades and didn't seem ready for this one despite all the warning signs," The Cook Political Report's Duffy said. "They haven't run the race they needed to run."
Negative advertising by Lugar has backfired in the local press partly because he has always been a more gentle campaigner.
Mourdock announced his candidacy in February 2011, and most Indiana Tea Party groups have coalesced around him. A Mourdock victory would be a coup for the populist Tea Party movement, its first significant Republican establishment scalp of 2012.
"If we beat Lugar it will show Republicans in Washington we are a force that is here to stay," said Greg Fettig of Hoosiers for a Conservative Senate, a Tea Party umbrella group backing Mourdock.
Conservatives failed at Utah's Republican convention in April to prevent Senator Orrin Hatch - also elected in 1976 and No. 2 in seniority behind Lugar - from running in a primary in June where his campaign fund of $5.7 million means he will be tough to beat.
(Additional reporting by Thomas Ferraro and Eric Johnson; Editing by Vicki Allen)