Citizens United paved the way for independent groups that could raise unlimited amounts of money from unions, corporations, and wealthy individuals to spend on elections, as long as they did not coordinate with individual candidates or parties. Republican donors moved quickly to seize on the ruling, pouring hundreds of millions of dollars into the last three national elections, while Democrats struggled to persuade their donors to invest in “super PACs” at the same scale.

Mr. Trump’s rivals for the Republican nomination were backed by an array of super PACs and outside groups set up by their allies and former aides, seeming to set the stage for even greater spending in 2016. But Mr. Trump unexpectedly triumphed, exploiting his knack for free publicity and relying on his own fortune and money from grass-roots supporters. In part because of that success, Mr. Trump was slow to incorporate super PACs into his general election strategy.

By contrast, Mrs. Clinton began personally courting donors for outside groups almost as soon as she entered the campaign in spring 2015.

The Democrats’ tentpole is Priorities USA, a five-year-old super PAC that has access to the party’s biggest donors and the implicit blessing of Mrs. Clinton, and is now on track to raise $173 million by Election Day. That is more than any equivalent Democratic effort in history, including the controversial big-money groups set up by wealthy liberals a decade ago to unseat President George W. Bush. The PAC is closely coordinating with environmental and labor activists and other organizations set up to harness support from veterans, African-Americans and Latinos.

In twice-monthly meetings at a Democratic law firm in downtown Washington, officials at Priorities have convened representatives of a dozen super PACs and progressive organizations to carve out swing-state turf and share intelligence from organizers on the ground. Several have pooled money with Priorities USA to purchase television and digital advertising through the same media firms, allowing smaller groups to get better rates. (Other left-leaning organizations, including labor unions and a super PAC founded by the billionaire environmentalist Tom Steyer, have separately spent significant money on field organizing.)

Priorities USA has shared the cost of some major campaigns with Emily’s List, the leading progressive group aimed at electing Democratic women. The super PAC has also given smaller groups access to its digital and video creative studio or shared its own television spots with other organizations to run under their own names.