Testerman told The Hill that she filed papers to run on Friday and that government overreach in the lives of Americans inspired her to enter the race.
"We want government off of our telephone lines, out of our computers. We want the government to quit monitoring everything that we're doing and dictating our every move and basically using government agencies to intimidate us," she said.
Testerman cited in particular a letter signed by Shaheen and a handful of other senators urging the Internal Revenue Service to investigate whether groups engaging in political activity were inappropriately granted a nonprofit designation.
Testerman suggested the letter inspired inappropriate scrutiny of conservative groups from the IRS.
Shaheen remains popular in early polling of the state and isn't considered vulnerable going into reelection, but she's already drawn two other Republican challengers: former state Sen. Jim Rubens; and Andy Martin, a conservative activist who has repeatedly run for office in multiple states over the past three decades.
Testerman said she decided to run because of the strong support she was seeing in New Hampshire.
She had planned to launch a campaign if she received 5,000 signatures on a petition to draft her into the race or if she received a pledge of $50,000 from donors.
While she didn't meet either goal, she said it was a "combination of both" that enticed her to run, as well as some guarantees from big donors in New Hampshire, though she declined to offer further details.
She'll carve out a conservative path to make it through the primary, she said, citing her role as founder of the New Hampshire arm of the Family Policy Council network.
And she charged that Shaheen's positions in favor of abortion rights and gun control have made her vulnerable.
"She's really very vulnerable as far as the Live Free or Die state is concerned," she said.
Testerman was in Washington from Tuesday through Sunday, meeting with lawmakers and conservative groups in the Capitol. On Friday, Testerman stopped by the Values Voter Summit, where she said she ran into Sen. Tim ScottTim ScottA better economic policy Republicans rebuke King for racial remarks Conway on criticism: 'I'm not there to read about myself' MORE (R-S.C.) before heading to Capitol Hill to meet with Sen. Kelly AyotteKelly AyotteFEC commissioner to Trump: Prove voter fraud Live coverage: Day three of Supreme Court nominee hearing Lewandowski saw no evidence of voter fraud in New Hampshire MORE (R-N.H.).