Dick Lugar has been in Washington a long time.

He's served in the U.S. Senate for six terms and 35 years, a run that may well come to an end if he loses today to his conservative primary challenger, state Treasurer Richard Mourdock.

If Lugar loses, he'll make history.

Lugar ranks among an elite, historical set of Senate old-timers. Only 22 senators in history served as long as Lugar has, out of 1,931 total, according to the Senate historian. Only 17 have served longer. Only one other six-term senator has lost in a primary: Kenneth McKeller, D-Tenn., who joined the Senate in 1917 and lost to Democratic primary challenger Al Gore Sr. in 1952.

Lugar currently ties Utah's Orrin Hatch as the Senate's longest-tenured Republican. Hatch, too, is facing a conservative primary challenge in 2012.

Lugar is also one of few senators left who's willing to cross the aisle. Lugar calls himself conservative, but he's a centrist by today's standards in willingness to be bipartisan.

Lugar voted to confirm President Obama's two Supreme Court nominees, Sonia Sotomayor and Elena Kagan; he voted to advance the DREAM Act; and he worked with the administration to ratify the latest START nuclear-arms-reduction treaty with Russia, even as other Republicans opposed it and as incoming freshmen GOP senators, elected in 2010, called on the Senate to wait until they got there to act.

If Mourdock wins tonight, and wins Lugar's Senate seat in the fall, the Senate will gain a tea partier who wants to eliminate five federal agencies plus the IRS, and who told ABC News in April, "Bipartisanship has taken us to the brink of bankruptcy … we don't need bipartisanship." Mourdock would join a growing coalition of senators who have, at times, resisted the current GOP leadership and have made the Senate GOP more conservative.

And if Democratic Rep. Joe Donnelly goes on to win the general election in November, Republicans may have lost a Senate seat because their primary upset the long-tenured status quo. Democrats, after all, have said they're more eager to face Mourdock in November than Lugar.