Sen. Richard BurrRichard BurrTop Senate Dem: ‘Grave concerns’ about independence of Russia probe Trump's pick for intel chief to get hearing next week A guide to the committees: Senate MORE (R-N.C.) will headline a fundraiser to boost North Carolina House Speaker Thom Tillis's (R) 2014 Senate bid, according to an invitation obtained by The Hill.

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Burr will join Sens. Bob CorkerBob CorkerA guide to the committees: Senate Republicans play clean up on Trump's foreign policy GOP Congress unnerved by Trump bumps MORE (R-Tenn.) and John BarrassoJohn BarrassoEPA head previously used private email for government business Big Pharma must address high drug prices A guide to the committees: Senate MORE (R-Wyo.) for the Sept. 24 fundraiser in Washington, D.C., a sign establishment Republicans are rallying around Tillis even as others get into the primary against him.

Burr's support is especially important; along with North Carolina Gov. Pat McCrory (R), he's the state's highest-profile Republican.

A source close to Burr, however, says the fundraiser is "not an official endorsement" of Tillis.

"He'll help any North Carolina Republican who wants to come to D.C. to raise money," the source says.

Tillis is considered the GOP front-runner to face Sen. Kay HaganKay HaganLinking repatriation to job creation Former Sen. Kay Hagan in ICU after being rushed to hospital GOP senator floats retiring over gridlock MORE (D-N.C.), a top GOP target. But he faces an expanding field of opponents, which could force him to run to the right and spend resources he'd prefer to save for the general election. 

Tea Party candidate Greg Brannon (R) has been in the race for months, and Baptist minister Mark Harris (R) told supporters last week that he'll run. North Carolina state Senate President Phil Berger (R) has also kept his name in the mix.

According to a Public Policy Polling survey last week, 43 percent of Republicans are unsure whom to support in the GOP primary. Berger had 13 percent support, while Tillis had 12 percent. 

But the support from Burr and other high-profile Republicans gives Tillis a major fundraising boost that could help him avoid a tough primary in the swing state.

According to PPP, Hagan leads Tillis by 15 percentage points — 51 percent to 36 percent — in a hypothetical general election match-up. 

Donations are set at $1,000 per political action committee and $500 per person to attend the Tillis fundraiser.

— Updated at 2:29 p.m.