During this year’s second quarter, Democrat Kay Hagan raised more than twice the money for her re-election campaign for one of North Carolina’s two seats in the U.S. Senate than her Republican challenger Thom Tillis, the speaker of the N.C. House.

Hagan’s campaign has reported that she raised $3.6 million from April to June, and she has $8.5 million cash on hand. Tillis’ campaign reported that he raised $1.6 million during the same period and has $1.5 million in the bank.

The candidates were required to submit their campaign finance data to the Federal Election Commission by July 15, but the commission has yet to publicly release their numbers. The Senate race in North Carolina might be the key to whether national Republicans regain their majority in the U.S. Senate after the fall elections.

Hagan’s campaign criticized Tillis’ ties to the Koch brothers, who have poured in millions of dollars in the race to air anti-Hagan ads on the state’s television stations. Tillis attacked Hagan for her ties with Tom Steyer, a billionaire political activist who wants to help Democratic politicians in the fall elections.

Pundits expect Hagan and Tillis to raise millions of dollars in their campaigns. Political action committees have spent several millions of dollars running attack ads on television and radio station in the state supporting and criticizing both candidates.

“Kay’s record-setting second-quarter funding total is a reflection of the enthusiasm and momentum behind her campaign,” Hagan campaign spokesman Chris Hayden said in a statement.

Jordan Shaw, Tillis’ campaign manager, said that the GOP hopeful remains on track with his fundraising efforts.

“Thom’s strongest fundraising quarter yet shows that North Carolinians want a proven problem solver to clean up the mess in Washington created by Kay Hagan and (President) Barack Obama over the last six years,” Shaw said. “It’s clear that we have the resources and the momentum needed to compete against Kay Hagan and the fringe liberal special interests trying to keep her Senate seat so she can continue to rubber-stamp Obama’s failed economic policies and out-of-control spending.”

John Dinan, a political-science professor at Wake Forest University, said he wasn’t surprised that Hagan is leading Tillis in political fundraising so far in the race.

“Incumbents usually have an overwhelming advantage in raising money from PACs, and they are often able to raise more from individual donors,” Dinan said. “These fundraising advantages are just some of the reasons why congressional incumbents are re-elected at a 90-percent rate most years.”

However, Tillis can unseat Hagan “either because they can overcome the direct-funding disadvantage through independent ad backing or because their messages and issues carry the day. It is too early to say which candidate in the North Carolina Senate race will benefit more from these independent ads.”

On other fronts, Hagan has agreed to three televised debates with Tillis this fall. Aside from the fundraising battles, the contest between Hagan and Tillis has become an intense affair with their campaign staffs focusing on the records of both candidates on issues such as national energy policy, the economy, the Affordable Care Act, abortion and the thorny issue of racial politics in the state.

Both campaigns are questioning their opponents’ ties with out-of-state political donors as most North Carolinians concentrate on summer activities such as their vacations, baseball games and picnics.

Hagan agreed to participate in three debates — fewer than the minimum of 10 that Tillis wanted for their fall campaigns.

This month, Hagan’s campaign said she accepted an invitation from the League of Women Voters and WECT-TV for a debate in Wilmington, the Associated Press reported.

Hagan already agreed to two debates by the N.C. Association of Broadcasters Educational Foundation. Tillis had already accepted invitations by the League and broadcasters’ foundation.

Hagan’s debate adviser wrote Tillis, saying three debates were held in the 2010 North Carolina Senate race.

Shaw, Tillis’ campaign manager, said it was “totally unacceptable” that Hagan wants just three debates, saying it’s limiting opportunities for the public to hear them. Shaw said Hagan should compromise and agree to six debates.

“We hope Sen. Hagan will stop attempting to deny voters a substantive discussion about her record and the future of our country,” Shaw said.

However, Hagan’s campaign staff told their counterparts in Tillis’ campaign that Republican Sen. Elizabeth Dole refused to debate Hagan in 2008. Hagan beat Dole to win her first term in the U.S. Senate six years ago.

At the same, Tillis’ campaign and the N.C. Republican Party continue their efforts to tie Hagan to Democratic President Obama and the Affordable Care Act, also known as Obamacare. Hagan’s critics point to her voting record as a senator that they said supports Obama’s legislative agenda in Congress.

Shaw said that Hagan is trying to distance herself from Obama.