Already a savagely personal race, the duel between Cochran and activist state Sen. Chris McDaniel could now drag on until the next vote on June 24 and present national Republicans with a dilemma: Whether to continue supporting the senator and tearing down McDaniel at the potential cost of damaging the party’s eventual nominee.

Outside groups have already spent more than $8 million in the Republican Senate primary, an extraordinary sum in a small state that rarely hosts competitive federal elections. Cochran and his allies have assailed McDaniel as a bumbling snake-oil salesman and finger-in-the wind opportunist who’s out of touch with Mississippi’s priorities. McDaniel and his campaign have attacked Cochran’s record of voting for federal spending, accused him of being soft on President Barack Obama and raised not-so-veiled questions about the senator’s age.

( Full 2014 election results)

All that may continue for weeks to come, with no easy way out for a national GOP that worked strenuously to bolster Cochran against his sharp-elbowed challenger.

With 98 percent of precincts reporting, McDaniel held less than a 1-percentage-point lead over Cochran, the second-longest serving Republican in the U.S. Senate. Neither candidate has won the simple majority needed to avert a second round of voting: At midnight, McDaniel had 49.6 percent of the vote to Cochran’s 48.8 percent, a difference of about 2,500 votes out of more than 300,000 cast.

An obscure third candidate, Thomas Carey, had 1.6 percent — probably just enough to prolong the political plight of Republicans in the state and nationally by three more weeks.

The primary was balanced on a knife’s edge in the run-up to June 3, as outside groups continued to plow hundreds of thousands of dollars into ads supporting both candidates. The Republican National Senatorial Committee rushed additional field staff to the state to fill gaps in Cochran’s turnout operation.

( Driving the Day: Primary preview)

And Cochran appeared to benefit from a wave of sympathy after a group of pro-McDaniel activists was arrested and charged with a lurid conspiracy to break into a nursing home and take photographs of the senator’s wife, Rose Cochran, who suffers from progressive dementia.

All that was not enough to propel Cochran across the finish line. It is now unclear which national groups would continue to spend millions on the runoff, or whether Cochran will continue to enjoy the foursquare support of Mississippi’s Republican establishment.

Cochran did not give a speech on election night. His campaign tweeted that the race was a “dead heat,” writing: “New campaign starts tomorrow. Three weeks to victory!” In his own election night remarks, McDaniel expressed confidence that he would emerge as the nominee, “whether it’s tomorrow or three weeks from tonight.”

( Also on POLITICO: Primary day: The 7 key questions)

“This is a historic moment in this state’s history. And because of your hard work, because of your dedication, we sit here tonight leading a 42-year incumbent,” McDaniel said.

Cochran backers acknowledged ahead of Tuesday’s vote that a runoff would be an alarming prospect, one that would likely force the senator to compete with an even smaller group of voters that skews still further to the right.

Democrats have watched the race as intently as Republicans: Despite Mississippi’s strongly conservative tilt, Democrats hope to mount a competitive general-election campaign against McDaniel, a slash-and-burn ideological activist who fashions himself after Texas Sen. Ted Cruz and has a hefty record of incendiary statements and personal associations.

Within the past two weeks, private Democratic polling has shown that the party’s nominee, former Rep. Travis Childers, would start a general election statistically tied with McDaniel. A race against Cochran, who is well-liked by independents and many Democrats, would be difficult to the point of futility.

Even if he was unable to capture his party’s nomination outright Tuesday, McDaniel’s upset showing is an agonizing blow to entrenched GOP leaders in Jackson and Washington D.C. — and a banner triumph for the national conservative groups that plowed millions into his campaign.

In a season of defeats for the activist wing of the Republican Party, McDaniel represents a powerful corrective to forecasts of the tea party’s demise. Though McDaniel reported raising only $1.3 million for his own campaign, Club for Growth put $2.5 million into boosting him; Senate Conservatives Fund spent more than $1 million as other spenders, including Tea Party Patriots and Citizens United, piled on.

Cochran enjoyed heavy-duty outside backing for his campaign, as well, including a $1.7 million effort by the Mississippi Conservatives super PAC, a group led by Republican National Committee member Henry Barbour and promoted by Haley Barbour, the former Mississippi governor. Business groups including the U.S. Chamber of Commerce and the National Association of Realtors added hundreds of thousands of dollars more to Cochran’s air support.

Now, all that spending may wind on for the better part of a month, costing millions of dollars more and likely intensifying already-bitter divisions the race has opened within the GOP.

Even with that looming risk, some influential GOP strategists would still favor an all-out war on McDaniel, whose record of controversial statements about Mississippi-centric issues, such as hurricane relief, and past incendiary remarks about immigration and homosexuality may make him a tough sell for middle-of-the road Republicans and the party’s major donors.