Democratic leaders from President Obama on down have assailed the spending by outside conservative groups, complaining that the business lobby and other Republican allies are pouring millions of dollars into the election as they shield donors from public scrutiny.

But now independent groups that support Democrats – including unions, environmental groups and new players – are fast gaining ground, often while using some of the same techniques condemned by Democrats. The strategies include using money from nonprofit organizations that can keep their donors secret and tapping into union dues to pay for unlimited attack ads.

Nearly four of every 10 dollars spent by independent groups last week was aimed at helping Democratic candidates, according to a Washington Post analysis of campaign-finance disclosure filings. The number is a dramatic improvement from just a few weeks earlier, when Democratic allies were being outspent by 7 to 1, the data show.