Republican U.S. Senate Minority Leader Mitch McConnell, right, and Democratic U.S. Senate candidate Alison Lundergan Grimes, left, prepare for a debate in Lexington, Ky., on Monday. (Pool/Reuters)

Key Democratic groups said Tuesday that they were abandoning the airwaves in the Senate battle in Kentucky, increasing the chances that Minority Leader Mitch McConnell (R) will hold on to his seat in November.

The groups say they have not reserved any advertising time for the final three weeks of the campaign in Kentucky, according to advertising documents and advisers in both parties. Instead, those resources will be used to shore up endangered Democratic incumbents in the most critical states — such as Colorado and Alaska — and for late-breaking races in Georgia and South Dakota.

Once expected to be the most expensive Senate contest in history, the race between McConnell and Democrat Alison Lundergan Grimes has appeared to tilt in the GOP’s favor in recent days after the challenger’s refusal to say whether she voted for President Obama.

Late Tuesday, after news broke of the decision to hold back advertising dollars, Democratic officials in Washington announced they signed off on $300,000 in funds to help get-out-the-vote efforts.

With Democratic advertising resources evaporating, just one Republican-leaning group has reserved advertising time for the homestretch, allowing other conservative committees to move their dollars elsewhere as well.

It was a fight night in Kentucky on Monday, as Democrat Alison Lundergan Grimes faced off against Sen. Mitch McConnell (R-Ky). The race is one of the most-watched Senate battles in the country. (Jackie Kucinich/The Washington Post)

Advisers in both parties cautioned that the Grimes-McConnell race is not over, with McConnell holding a steady lead of several percentage points in most polls. But a liberal strategist suggested that the Kentucky voters have hardened their positions on the race and that moving the numbers would take a significant investment of dollars that cannot be justified given opportunities in other races.

It’s a surprising shift for the high-profile Democratic bid to knock off McConnell, which has drawn spending from at least 25 outside groups, according to a spreadsheet of advertising dollars spent this year. The spending outline was provided by a Republican but the content was largely confirmed by a Democrat involved in media spending.

Total television and radio advertising in the race, including reserved time for the final three weeks, will hit almost $48 million. That’s a staggering sum for a relatively small state. That tally also does not include some money spent during the Republican primary campaign.

But now the two candidates will have the airwaves largely to themselves. Their overall spending plans, which could be altered depending on the contours of the campaign, are fairly even.

Grimes, the Kentucky secretary of state, is slated to spend about $1.4 million on TV and radio ads in the three largest markets of Cincinnati, Lexington and Louisville, while McConnell’s campaign has reserved about $1.5 million in those same markets.

Grimes’ team remained optimistic Tuesday that the race was still winnable. They noted energy on the ground, pointing to a recent 3,000-person rally in sparsely populated southwest Kentucky, and they suggested that the race would still pivot on the future-versus-the-past theme that Grimes, 36, repeatedly laid out in her debate with McConnell, 72.

Most strategists in both parties had envisioned a race in which the outside groups would pour money relentlessly into the state’s airwaves until Election Day. Democrats had made knocking off McConnell a top priority.

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On Tuesday, officials with the Democratic Senatorial Campaign Committee, the official campaign arm, and with Senate Majority PAC, an outside group closely affiliated with the leadership, acknowledged that they had not reserved any advertising in Kentucky for the next three weeks.

The two groups had aired close to $6 million worth of ads between them, and had spent plenty more on voter turnout, polling and research.

In a two-week period in late September and early October, more than 11,000 ads about the Senate race aired on broadcast networks in the state, costing more than $5.6 million, according to a study by the Wesleyan Media Project.

Only in North Carolina and Iowa were more ads run by candidates and outside groups.

The United Mine Workers, which endorsed Grimes in early August, said it will continue its work on her behalf with a modest advertising campaign. On McConnell’s side, a committee dedicated to his reelection — established by former aides and accepting unlimited donations — is expected to stay on air through Nov. 4.

In Georgia, the DSCC will spend $1 million on ads in the race against GOP Senate nominee David Perdue, who has come under fire for a remark in a 2005 deposition that he spent “most” of his private-sector career on outsourcing jobs. Democratic nominee Michelle Nunn has pounced on the comment in two TV ads.

Republicans need to gain six seats to achieve a Senate majority. Democrats have basically surrendered in Montana and West Virginia, and they are on defense in Arkansas, Louisiana, Alaska, Colorado, Iowa and North Carolina.

Grimes has found herself on the defensive about her refusal to say who she voted for in the 2008 and 2012 general elections for president. The question was raised last week in a newspaper editorial board meeting and again Monday in a debate with McConnell.

She has repeatedly called herself a “Clinton Democrat” and noted her support for Hillary Rodham Clinton’s presidential bid. But she declined to say whether she voted for Obama in 2008 or in 2012, when she attended the nominating convention as a voting delegate.

At Monday’s debate — the race’s only one-on-one encounter in such a setting — she explained her decision by saying she was “not going to compromise a constitutional right” to privacy at the ballot box.

“The president is not on the ballot this year. It’s myself and Senator McConnell, and he doesn’t want to take responsibility for all that’s wrong in Washington, D.C.,” Grimes said.

The answer was widely panned by commentators on both sides as a transparent attempt to avoid being linked with Obama, who is deeply unpopular in the state.

On Tuesday, officials at the Fayette County clerk’s office confirmed that Grimes has voted in every primary and general election going back to at least 2008.

sean.sullivan@washpost.com