MANCHESTER, N.H. — A Democratic super PAC wanted to run a 30-second ad during an Oct. 31 evening newscast on New Hampshire’s one network station – and was even willing to shell out the $10,000 that the station demanded.

Hours before it was set to run, however, WMUR-TV had to revise its contract with Senate Majority PAC and credit the group’s account. The reason: “Oversold inventory.”

Such is the life of even a deep-pocketed political action committee at this late stage of the 2014 campaign. Many of these groups want to keep spending in a final push before the Nov. 4 elections, as Democrats defend their Senate majority and Republicans drive for the six seats required to command it.

But often there’s simply no ad time left.

“Campaigns and third-party groups are finding it difficult to even find spots to purchase,” said Isaac Baker, who was a top official on the advertising team for President Barack Obama’s re-election bid and now advises some of the Democrats’ heaviest spenders. “And they’re seeing rates climb from where they were last year or even earlier this year.”

New Hampshire’s Senate race – which includes media markets in Maine and Massachusetts – is seeing, on average, a three-fold increase in overall spending on political advertising. And prime ad time is virtually saturated.

In the past two weeks alone, Manchester’s WMUR-TV told political groups that their ads during Good Morning America, General Hospital and Jimmy Kimmel Live, among others, could not be aired because the station over-sold chunks of time, according to contracts filed with the Federal Communications Commission.

“The sell-out rates are high and … television remains king in political races,” said Joy Baer, President of the Chicago-based advertising technology firm STRATA.

Campaign spending in New Hampshire is hardly the most intense in the nation at this late moment.

Spending in North Carolina’s Greenville market is up 15 times from what it was a year ago. Iowa’s Sioux City market has a 14-times increase. Colorado Springs? That’s an 11-fold increase, according to STRATA data.

Rates in Anchorage, Alaska, are on par with those in Chicago.

The reason: Campaigns and candidates in these hard-fought races booked airtime in the spring, leaving little inventory for late-notion buys. For those determined to find last-minute time, it can be had – but at a steep increase from normal rates.

For instance, Senate Majority PAC signed a contract late last week to snag one minute of time during the Oct. 27 Dancing With the Stars  for $24,000 as polls showed Democratic Sen. Jeanne Shaheen stuck in a tight race with her Republican challenger, former Massachusetts Sen. Scott Brown.

Campaign ad rates operate in two systems. Candidates themselves are guaranteed the lowest available rate under federal finance laws, but their allies and super PACs are forced to compete with each other, as well as commercial advertisers such as Coca-Cola or Target.

Campaign committees can avoid deep gouging by planning ahead. For instance, the Democratic Congressional Campaign Committee reserved its advertising time in May, and the National Republican Congressional Committee followed in June. As a result, they locked in their rates and saved as much as 35 percent.

The spending might be for naught.

Kyle Karamanoogian, a 31-year-old technology worker whose office is down the street from WMUR-TV’s Manchester studios, said no ad was going to sway him. The Bedford resident considers himself a Republican-leaning independent but has yet to settle on a candidate in the state’s Senate race.

“I laugh. I think they’re all terrible,” he said. “They’re like a bunch of middle-schoolers talking at the lunchroom table.”

The intense advertising he’s talking about is a conundrum for political ad makers. Adding millions to an already busy market runs the risk of simply wasting money that could be used for phone calls, door knocks or direct mail.

New Hampshire’s Senate race has already drawn $45 million in spending. Almost $12 million of that has been in the form of ads.

“There is definitely a sense of diminishing returns when it comes to the arms race of TV advertising,” said Baker, the Democratic ad adviser.

That doesn’t mean the groups are surrendering. One conservative super PAC, Ending Spending Action Fund, is spending more than $1 million for ads during the final five days of the New Hampshire campaign. The group had to reserve its time a month ago to make it to the air at all, even with the steep price.

The PAC’s President, Brian Baker, said his ad buyer spotted “one last slot” available during this week’s Patriots game on the Boston CBS affiliate and snatched it up. The price for a 30-second ad: $100,000.

“Exposure to the target audience of likely swing voters: priceless,” Baker said.

(HOLLY RAMER and PHILIP ELLIOTT)