CONCORD, N.H. — Democratic New Hampshire Gov. Maggie Hassan announced Monday she will challenge Republican Sen. Kelly Ayotte, setting the stage for a highly competitive race that will be critical in determining majority control of the Senate in the next president’s first year.

In an online video, Hassan said she’s running to bring a bipartisan, common-sense governing approach to Washington that mirrors that of New Hampshire. She said Washington has given in to powerful special interests and lobbyists “who rigged the system for themselves and against the middle class.”

Hassan, a two-term governor, has long been considered the Democrats’ best chance of beating Ayotte, a former attorney general who won her seat during the 2010 Republican wave election.

Democratic voter turnout spikes in New Hampshire in presidential years, handing the party a win here in 2008 and 2012. Democrat Hillary Clinton, the party’s front-runner for the presidential nomination, won the 2008 primary and has long been popular in the state, marking another potential hurdle for Ayotte.

Hassan, 57, is one of the last major Democratic recruits to jump into the 2016 race. New Hampshire joins Illinois, Wisconsin, Pennsylvania and several other states as keys to control of the Senate, and the race between Hassan and Ayotte is likely to cost tens of millions of dollars.

In a statement, Ayotte said she expects a spirited campaign and is seeking re-election to fight for “better opportunities and a brighter future for our kids and our state.”

Hassan’s announcement comes just more than two weeks after reaching a deal with legislators allowing an $11.3 billion budget plan to go into effect after a months-long stalemate with Republican lawmakers over business tax cuts.

Hassan comfortably won re-election in 2014, campaigning on a message of a bipartisanship that hinged on the state budget and passage of Medicaid expansion in early 2014. More than 42,000 people signed up for insurance under the expansion, but the law will sunset at the end of next year if lawmakers don’t vote to reauthorize it in the upcoming session.

A number of accomplishments Hassan outlined in the video came during her first term in office, including freezing tuition at state universities, lowering it at community colleges and doubling the research and development tax credit.

In her video, Hassan said lawmakers can “do better” for people struggling to pay for college, police officers fighting drug abuse and “thousands of women who’ve had access to health care services threatened by politicians playing games with Planned Parenthood.”

Women’s issues are likely to be a major focus of the 2016 campaign. Ayotte does not support government funding for Planned Parenthood, which has been questioned in recent months for providing fetal tissue from abortions for research. But Ayotte warned her fellow Republican lawmakers last week against shutting down the government over funding for the organization.

Ayotte launched her re-election bid during the summer. One of a few female Republican senators, Ayotte rose quickly to national prominence and has become a leading Republican voice on national security and other issues. She is often mentioned as a potential vice presidential candidate.

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