Carly Fiorina declared her candidacy for the U.S. Senate–in a bid to replace another well-known woman, incumbent California Democrat Barbara Boxer.

Fiorina, who was No. 1 on Fortune‘s Most Powerful Women list for six years when she was CEO of Hewlett-Packard , will be pounding the campaign trail simultaneously with another ex-No. 1 on our list: Meg Whitman. The former eBay CEO, who topped Fortune‘s power list in 2004 and ’05, is running for Governor.

Neither woman, both Republicans, will have an easy time in the left-leaning, financially crippled Golden State. Running on her “I’m a great manager” platform, Whitman has a decent shot at her party’s nomination. But she faces a fierce Democratic rival in Jerry Brown, California’s current Attorney General who once was Governor. Another Democratic rival, San Francisco Mayor Gavin Newsom, just dropped out. (For more, check out my recent cover story, “Can Meg Whitman Save California?“)

Fiorina, who yesterday revealed her plans in the
Orange County Register
, has a personality tailor-made for campaigning: She’s charismatic and commanding. Remember when she was waging that brutal proxy fight to buy Compaq in 2002? She played it like a political candidate–and she won.

But Fiorina, 55, who worked with Whitman on John McCain’s failed Presidential campaign, carries significant baggage into this latest race: She was fired by the H-P board in 2005–as much for her style of leadership as her disappointing execution.

Another battle lately has been a medical one. Fiorina was recently treated for breast cancer. In September, while undergoing daily treatments at Stanford Hospital, she spoke by video-conference, along with Elizabeth Edwards, to participants of the Fortune Most Powerful Women Summit. Here’s a clip:

Kudos to Fiorina for speaking out. The fact that she’s running for the U.S. Senate is a sign that her prognosis is good. And she’s as tenacious as ever.