Only in the last two weeks or so have Democratic-oriented groups finally begun to come close to matching the spending of their counterparts on the right. But in many cases they appear to be playing defense, rushing to bolster Democratic candidates in races in which Republican outside groups had been swamping them.

America’s Families First Action Fund, for instance, a new Democratic-aligned group that is able to accept contributions of unlimited size from individuals and corporations but regularly reports its donors to the election commission, has emerged in the last few weeks as a major player. But in almost all of the races it has been involved in, it is mostly laboring to keep up with Republican outside group spending.

Last week, for example, the group spent $362,000 on a television ad attacking Steve Southerland, the Republican challenger to Representative Allen Boyd, Democrat of Florida. But the 60 Plus Association, a nonprofit advocacy group that bills itself as a conservative alternative to AARP, began attacking the Democratic incumbent on television as early as late August.

In the closing stretch of the campaign, Democratic candidates in competitive races generally have had more money in the bank to spend than their Republican counterparts. As of Oct. 13, Democrats in House races in play collectively had about $45 million in cash on hand, compared with about $32. million for Republicans.

In contest after contest, however, Democratic candidates with huge financial advantages over underfinanced Republican opponents have found themselves under siege.

Outside group spending has already far exceeded the total for the last midterm election cycle, in 2006, and is on track to surpass even what was spent by independent groups in 2008, a presidential election year, according to data from the Center for Responsive Politics.

The hand-to-hand political combat between candidates, who must inch along in their own fund-raising in relatively modest bites, and these groups, which are able to leapfrog ahead with the help of a single giant donation, casts in bold relief the kind of outsized influence corporate and individual megadonors to such organizations can exert on specific races.