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PACs backing Mr. Romney and Republican candidates for the Senate. Joe Ricketts, the owner of the Chicago Cubs, spent close to $13 million to bankroll a super PAC attacking Mr. Obama over federal spending.

Bob Perry, a Texas homebuilder, poured more than $21 million into super PACs active in the presidential race and the Senate battles in Florida and Virginia, where Democrats narrowly prevailed.

A donor network marshaled by Charles and David Koch, the billionaire industrialists and conservative philanthropists, reportedly sought to raise $400 million for tax-exempt groups that are not required to disclose their spending.

Mr. Adelson's giving to super PACs and other outside groups came to more than $60 million, though in public Mr. Adelson did not seem overly concerned about the paltry returns on his investment.

"Paying bills," Mr. Adelson said Tuesday night when asked by a Norwegian reporter how he thought his donations had been spent. "That's how you spend money. Either that or become a Jewish husband - - you spend a lot of money."

Flush with cash, Republican-leaning groups outspent Democratic ones by an even greater margin than in 2010. But rather than produce a major partisan imbalance, the money merely evened the playing field in many races.

In several competitive Senate races, high spending by outside groups was offset to a large extent with stronger fund-raising by Democratic candidates, assisted at the margins by Democratic super PACs. For much of the autumn, Mr. Obama and Democratic groups broadcast at least as many ads, and sometimes more, in swing states than Mr. Romney and his allied groups, in part because Mr. Obama was able to secure lower ad rates by paying for most of the advertising himself. Mr. Romney relied far more on outside groups, which have to pay higher rates.

Haley Barbour, a former Mississippi governor who helped Mr. Rove raise money for American Crossroads and its sister group, Crossroads Grassroots Policy Strategies, said that without a blitz of coordinated anti-Obama advertising in the summer, the campaign would not have been as competitive.

"I believe that some of that money actually kept Romney from getting beat down by the carpet-bombing he underwent from the Obama forces," Mr. Barbour said."I did look at it more as us trying to keep our candidates from getting swamped, like what happened to McCain."

Some advocates for tighter campaign financing regulations argued that who won or lost was beside the point. The danger, they argued, is that in the post-Citizens United world, candidates and officeholders on both sides of the aisle are far more beholden to the wealthy individuals who can finance large-scale independent spending.

"Unlimited contributions and secret money in American politics have resulted in the past in scandal and the corruption of government decisions," said Fred Wertheimer, the president of Democracy 21, a group that monitors campaign spending. "This will happen again in the future."

But on Wednesday, at least, U.S. megadonors returned home with lighter wallets and few victories.

As the morning wore on at Logan Airport, more guests from Mr. Romney's election-night party at the Boston Convention and Exhibition Center trickled in, lugging garment bags and forming a small line at the security checkpoint.

"It's going to be a long flight home, isn't it?" said one person, who asked not to be identified.

The investor Julian H. Robertson Jr., who held fund-raisers for Mr. Romney and gave more than $2 million to a pro-Romney super PAC, arrived with several companions. Mr. Robertson spotted an acquaintance: Emil W. Henry Jr., an economic adviser and a fund- raiser for Mr. Romney, to whom Mr. Robertson had offered a ride on his charter.

"Aww, group hug," Mr. Henry said.