Thom Tillis, the speaker of the North Carolina House who was the Republican frontrunner to take on Democratic Sen. Kay Hagan in the fall, comfortably met the threshold to avoid a runoff in his party's primary contest Tuesday.

Tillis, who needed to garner at least 40 percent of the Republican vote, had received 45 percent with nearly two-thirds of precincts accounted for. Greg Brannon, a physician, was in second place with nearly 28 percent, followed by Mark Harris, a Baptist pastor, with roughly 17 percent.

Now, the match-up between Tillis and Hagan promises to be one of the most closely watched of the 2014 midterm election cycle, and could decide whether Republicans take control of the Senate.

National Republicans will hold up Tillis' victory Tuesday as an example of how to engineer desired electoral outcomes in party primaries. But getting to this point was not easy.

Despite Tillis being the favored candidate of national Republicans, and although leading Republican senators publicly voiced their support for Tillis and hosted fundraisers in Washington on his behalf, Tillis continued to fall short of the run-off threshold in most public polls well into April.

But Tillis raked up endorsements from groups across the Republican spectrum, including National Right To Life, the U.S. Chamber of Commerce, and the National Rifle Association. Tillis touted support from Mitt Romney and former Florida Gov. Jeb Bush. And the Karl Rove-backed super PAC American Crossroads spent roughly $1.6 million on television ads to boost Tillis in the final five weeks of the election alone.

Challengers to Tillis, namely Brannon and Harris, did not have big-money Tea Party groups backing them, although they did have some boldfaced names in their corners. Former Arkansas Gov. Mike Huckabee, a potential 2016 presidential candidate, gave Harris his stamp of approval, while Sen. Rand Paul, R-Ky., backed Brannon, even campaigning for him Monday in Charlotte.

But even Hagan and Democrats were not optimistic that either Brannon or Harris would hold Tillis to a run-off, and went straight to bombarding Tillis in television and radio ads.

Speaking to reporters after a debate last month, Tillis said Democrats were "trying to do everything they can at least to force a run-off, but I don’t even believe that’s going to happen."

On Tuesday, Democrats attributed Tillis' primary victory to the infusion of money from Republican groups into the race.

“Speaker Tillis’ Washington special interests succeeded in dragging him across the finish line, but they’ll find that defending his record in the fall is much more difficult than brushing off underfunded challengers," Ben Ray, a spokesman for Forward NC, a state Democratic outfit, said Tuesday.

For its part, American Crossroads blamed Democrats' meddling for the group's own intervention in the primary. “We engaged early and worked closely with other center-right groups to help Tillis overcome a late dirty-tricks campaign orchestrated by Hagan and national Democrats, " Stephen Law, the group's president and CEO, said in a statement.

The primary contest in North Carolina marked an important test case for the national Republican establishment, which during the 2012 election cycle saw multiple competitive Senate races derailed after weak candidates won GOP primaries.

In those primary contests, the National Republican Senatorial Committee and moderate Republican outside groups chose not to play favorites, preferring instead to conserve resources for the general election. In North Carolina this year, they took the opposite tack — with the desired result.