Credit: John Flavell/AP Photo, Tom Williams/Getty Images

Senate Minority Leader Mitch McConnell got a challenger for his U.S. Senate seat on Monday and her name is Alison Lundergan Grimes.

"I have met with my supporters. We have had a great conversation and determined and decided that we can next make the best move, the best difference in the commonwealth of Kentucky by running for the U.S. Senate," Grimes said at a news conference in Frankfort , Ky.

"Over the course of the past 12 weeks I have taken the time necessary to gather all the facts to make truly an informed decision and that includes listening to my supporters all across this state," Grimes said. "Make no mistake members of the media this due diligence was not reluctance, it was not hesitancy, but rather a deliberate gathering of all the necessary facts to make a decision that should not be taken lightly. During this process the question never was: 'Is Mitch McConnell vulnerable? Does Kentucky deserve a change?' The answer to both of those two questions remains and is yes. The question before my supporters which we have been working diligently on is, 'How best can we continue to make a difference and move this commonwealth forward.".

Grimes said she is "no stranger to being an underdog" and McConnell's ads already running "are based out of fear, of losing his 30 year grip on power and this Kentucky woman does not believe the voters of Kentucky will be fooled that easily."

The announcement from Grimes, the Kentucky secretary of state who is not a national figure like her opponent, was highly anticipated since the 34-year-old is thought to be the Democrats' best chance at defeating McConnell. Local polling has shown the Kentucky Republican is vulnerable, but until Monday, no high-profile Democrat had mounted a challenge.

"Accepting the invitation from countless Washington liberals to become President Obama's Kentucky candidate was a courageous decision by Alison Lundergan Grimes and I look forward to a respectful exchange of ideas," McConnell said in a statement shortly after the announcement.

Grimes' spokesperson Jonathan Hurst told ABC News earlier today that the announcement was not "an official roll out," and that an actual campaign kick-off would come later. At the event, Grimes said she would be forming the campaign over the next two weeks.

National Democrats responded to Grimes' decision with Guy Cecil, executive director of the Democratic Senatorial Campaign Committee, saying in a statement that the "race is now a toss up."

"Mitch McConnell is the most unpopular incumbent in the entire country," Cecil said. "He is a relic of the past and a symbol of everything that is wrong with Washington. Kentuckians want a change…We expect to preserve our majority next year by defending strong incumbents and playing offense in Kentucky and Georgia. We have a long road to travel, but Republicans are making our job a little easier each day, failing to recruit strong candidates and expand the playing field."

National Republicans weighed in on the news before Grimes officially announced with National Republican Senatorial Campaign Committee strategist Brad Dayspring saying in a statement before her announcement, "If this is a rollout to a Senate campaign it's amongst the worst in history. It is a total fumble to launch just as folks prepare for a holiday week."

In a preview of how Republicans will frame Grimes, Dayspring said the announcement is "even more bizarre for Alison Grimes in Kentucky, who watched the president that she nominated at the Democratic National Convention last summer just declare a war on coal and Kentucky families."

Ashley Judd is the most high-profile Democrat who took a pass on the race, announcing in March she would not run after seriously considering a bid. In May, Judd's most vocal supporter, Rep. John Yarmuth, D-Ky., urged Grimes to make an announcement soon saying it was "very important to do it now" and there were "others waiting in the wings" and Democrats want to "avoid an expensive primary."

After the news Yarmuth immediately responded saying in a statement, "I'm excited that Alison has decided to make the race. She will be a formidable candidate and a great contrast to Sen. McConnell. If she is our party's nominee, she will have no more enthusiastic supporter and tireless campaigner than me."

In March, Grimes met with former President Bill Clinton, a close family friend who encouraged Grimes to consider taking on McConnell, assuring he would support her. Clinton told Grimes then that she has "unlimited potential."

The Clintons are longtime friends and allies of Grimes's father, Jerry Lundergan, a former state party chairman, and Grimes herself who became secretary of state in 2011 after beating her primary challenger who was backed by Democratic Gov. Steve Beshear. Jerry Lundergan was a strong supporter of Bill Clinton, but also of Hillary Clinton's presidential bid in 2008, and they remain close.

Despite there being no marquee Democrat in the race until now, there are already ads on both sides.

A pro-McConnell super PAC Kentuckians for Strong Leadership released ads earlier this month, an answer to two Democratic groups that aired a volley of TV commercials earlier this month attacking McConnell.

According to a strategist with the group, Kentuckians for Strong Leadership spent $260,000 on the ads, which will run on broadcast and cable in the state. That's roughly $10,000 more than what two Democratic groups, the Senate Majority PAC and an allied organization, Patriot Majority USA spent on their ad.

As far back as February, the McConnell campaign began running online videos, and later a series of television ads, in support of his re-election bid even though he does not yet have a viable Democratic opponent.

Former Miss America Heather French Henry is still considering jumping into the race, as is Bill Garmer, an attorney and former state Democratic Party chairman, and Tom Fitzgerald, an environmental attorney, but this news is sure to affect their decisions.

There are three lesser known Democrats who have formally declared their candidacies: University of Louisville professor Greig Leichty, Louisville music promoter Bennie J. Smith, and building contractor and former congressional candidate Ed Marksberry.

This story has been updated since it was first posted.