WASHINGTON — Richard Lugar, the longest-serving Republican currently in the US Senate, lost to a more conservative, tea-party-backed candidate in Indiana's primary, US media projected.

With two-thirds of precincts reporting, challenger Richard Mourdock, the Indiana state treasurer, was beating Lugar -- an 80-year-old Republican moderate and two-time chairman of the Senate Foreign Relations Committee -- by 61 percent to 39 percent, CNN and NBC News forecast on Tuesday.

The 36-year veteran of the Senate, accused by critics of being too disconnected from Indiana voters, failed badly to recognize his own vulnerability from the party's more conservative flank until it was too late.

His fellow Republicans in the Senate wasted little time in calling on conservatives to move beyond the primary battle and back Mourdock against a Democratic challenger in the November election.

"It's time to thank Sen. Lugar for his years of service & throw all our support behind Richard Mourdock," conservative Senator Jim DeMint wrote in a Twitter message, calling the challenger a "principled conservative leader."

Lugar shares the title of longest-serving Republican currently in the Senate with Orrin Hatch of Utah, who himself is facing a primary challenge this year.