JACKSON (AP) — Six-term Sen. Thad Cochran defeated challenger Chris McDaniel in Mississippi’s Republican primary runoff Tuesday, successfully expanding the original primary electorate to deal another blow to tea party conservatives in a midterm election year dominated mostly by traditional Republican powers.

The 76-year-old senator called it a “great victory” for Mississippi, but McDaniel notably didn’t concede late Tuesday, raising questions about whether the 41-year-old state lawmaker will challenge the outcome.

He cited, but did not detail, “irregularities” that he attributed to “liberal Democrats” supporting Cochran.

“We are not prone to surrender, we Mississippians,” McDaniel said. “Before this race is over we have to be absolutely certain the Republican primary was won by Republican voters.”

Unofficial returns showed Cochran, who was first elected to Congress in 1972 and to the Senate in 1978, did indeed expand the electorate from the June 3 primary. With 99 percent of precincts reporting late Tuesday, Cochran held a lead of about 6,600 votes, with 50.9 percent of the vote to McDaniel’s 49.1 percent.

McDaniel had once been viewed as archconservatives’ best shot at knocking off a high-ranking Republican lawmaker, and he appeared to have momentum after leading the first round and then watching little-known tea party challenger David Bart defeat House Majority Leader Eric Cantor in Virginia’s Republican primary.

The race attracted about $12 million in spending from outside groups, as the matchup became a proxy for the internal Republican struggle between tea party conservatives and the party establishment.

At home it became a referendum on Cochran’s influence as a member and former chairman of the Senate Appropriations Committee. Cochran, who could become chairman again if Republicans win a Senate majority, promised throughout the campaign to “do more for Mississippi.”

McDaniel, an Ellisville attorney and former radio host, hammered Cochran as the face of a profligate Congress the nation can no longer afford.

Speaking Tuesday night, he renewed his charge that the Cochran campaign lacks the principle of true conservatism.