PETERBOROUGH, N.H. — Sen. Kelly Ayotte will endorse presidential candidate Mitt Romney Sunday, becoming the first congressional member in the state to throw her weight behind a GOP candidate, ABC News has learned.

In an e-mail tonight to supporters, Ayotte wrote, “I have been looking for a candidate that can lead our country based on our shared conservative principles and who can win.

“For me, Mitt Romney is that candidate and that is why I will be working as hard as I can to help him secure the Republican nomination and most importantly, ensure that Barack Obama is a one-term president,” Ayotte wrote.

“I believe Governor Romney will run a campaign against President Obama that is serious, substantive, and one we can be proud of,” she added.

New Hampshire’s Union Leader was the first to report the endorsement.

Ayotte, widely considered to be the top elected official in New Hampshire, is scheduled to attend a rally with Romney Sunday in Nashua where she will make the endorsement official.

According to the Union Leader, Ayotte is the first GOP senator from an early primary or caucus state to make an endorsement.

Ayotte joins the ranks of Romney backers in the state, including former Gov. John H. Sununu and former Sen. Judd Gregg, as well as several other state representatives.

Ayotte will be named co-chair of Romney’s national advisory committee and will campaign for him in the Granite State and other early states, according to the campaign.