WASHINGTON — In the Senate, Republicans lost ground, pouring well over $100 million in outside money into a half-dozen seats that went to Democrats. In the presidential race, GOP nominee Mitt Romney and his allies spent more than twice as much as John McCain in 2008, but only took back red-leaning Indiana and North Carolina for their trouble.

Even in the House, where last-minute surges of cash would seem to stand a good chance of swinging races, GOP money groups struck out repeatedly, according to the Post analysis. In 26 of the most competitive House races, Democratic candidates and their allies were outspent in the final months of the race but pulled out a victory anyway. That compares to 11 competitive races where Republicans were outspent and won.

Outside money was the dog that barked but did not bite. President Barack Obama and other Democrats had long made dire predictions about the potential impact of Citizens United v. Federal Election Commission, which allowed corporations and unions to spend unlimited funds on elections and created an entirely new class of wealthy political groups.

The money did dramatically change the focus and character of many campaigns. Candidates up and down the ballot were forced to spend more time than ever raising donations, while political advertising funded by outsiders was even more negative than before.

“Its lasting impact will be that it fueled the public’s disgust about politics,” said David Donnelly of the Public Campaign Action Fund, which favors stricter campaign-finance regulations.

Yet super PACs and secretive nonprofit groups — which spent up to $10 million a day on the presidential race alone — couldn’t move the needle far enough to prevail in nearly any of the big races they targeted. Outside money allowed Romney to be competitive with Obama, but that meant the candidate had no direct control over much of the spending.

In the end, the two sides reached a kind of dreary equilibrium, clogging the airwaves with so many attack ads that Republican groups began airing spots in California and other deep-blue states where they had little chance of victory. By the end of October, more than a million commercials had been broadcast in a presidential race that remained close to a dead heat for much of the year.

Indeed, if election investments are like the stock market, a lot of billionaires just lost their shirts. The two biggest pro-Republican groups, American Crossroads and Restore Our Future, together spent more than $400 million on federal races with relatively little to show for it in the end.