Liz Cheney announced today that she was withdrawing from the Senate race in Wyoming, abruptly ending her Republican primary fight against Sen. Mike Enzi, a three-term incumbent who became a far more formidable rival than she had anticipated when she declared her candidacy last summer.

"Serious health issues have recently arisen in our family, and under the circumstances, I have decided to discontinue my campaign," Cheney said in a statement. "My children and their futures were the motivation for our campaign, and their health and well-being will always be my overriding priority."

The health problems involve one of her teenage daughters, who has been diagnosed with diabetes, Republicans close to the family told ABC News. Several people familiar with the campaign in Wyoming and Washington said Cheney reached her decision over the holidays, and only began telling advisers in recent days.

Former Sen. Alan Simpson, a Wyoming Republican and longtime friend of the Cheney family, told ABC News that he spoke to Liz Cheney today and she characterized her withdrawal as a deeply personal family decision.

"She told us she was stepping away, that it was a mom thing," Simpson said in an interview from Wyoming. "Her children she felt needed her more - and she needed them."

But campaign officials said she had become increasingly frustrated with the trajectory of her campaign, and her inability to drive Enzi from the race or slow his fundraising.

"She knew she wasn't on a path to winning," one Republican official familiar with the campaign told ABC News. "She knew she couldn't afford a big loss."

Cheney, the 47-year-old daughter of former Vice President Dick Cheney, moved her family from Virginia to Wyoming in 2012 with an eye on a Senate seat in the state she was born in and that her father represented in Congress.

Her daughter's health problems accelerated the decision to re-evaluate her candidacy, Republicans close to the family said, along with the realization that her Senate bid had become an uphill fight.

"As a mother and a patriot, I know that the work of defending freedom and protecting liberty must continue for each generation," Cheney said in a statement this morning. "Though this campaign stops today, my commitment to keep fighting with you and your families for the fundamental values that have made this nation and Wyoming great will never stop."

Her campaign said she would have no comment today beyond the brief statement.

The announcement to abandon her Senate bid came almost as abruptly as her decision to enter the contest last July, when she surprised party leaders in Wyoming and Washington by setting up a contentious primary fight against Enzi, a well-respected conservative Republican. She sought to rally the support of her father's old political network in Wyoming but struggled to build a new network of her own and introduce herself to Wyoming voters. She also failed to win over many state or national Republican leaders, who sided with Enzi.

Her candidacy also sparked controversy inside her own family when she declared her opposition to same-sex marriage. Her younger sister, Mary Cheney, who is lesbian, forcefully challenged Liz's position and suggested that she had changed her views. The fight even drew in their parents, the former vice president and his wife, Lynne, who said they were "pained" to see the private rift become so public, but they ultimately sided with Liz.

Liz Cheney fought perceptions that she was a political carpetbagger after moving from the Virginia suburbs of Washington to Jackson Hole, Wyo. She aired campaign commercials to highlight her family's deep roots in the state but struggled to find an issue and gain traction challenging Enzi. But she faced one stumble after another, including when she applied for a fishing license in the state and claimed on her application that she had lived in Wyoming for 10 years. The episode became something of a punch line in Wyoming.

Her decision to leave the race was first reported by CNN Sunday night. While the Wyoming race did not affect the balance of power in the Senate, where Republicans are trying to seize the majority from Democrats, it does eliminate one divisive primary fight and could embolden establishment Republicans facing primary fights elsewhere.