In the final weeks of the 2014 campaign, Democrats are increasingly reliant on super PACs and their deep-pocketed donors to underwrite their effort to maintain their Senate majority. Republicans meanwhile, have money left in the party bank and a clear advantage when it comes to dark money — funds that can’t be traced to donors. These are just some of the insights we gained by visualizing data on key Senate races in a series of charts. The information comes from the Federal Election Commission via our Real-Time Federal Campaign Finance tool, which makes the up-to-the-minute campaign finance data available for easy analysis.

MORE: Confused about all the different kinds of election spending? Sunlight Editorial Director Bill Allison explains it all.

Another striking trend revealed this exercise: For all intense speculation about the fate of Senate Republican Leader Mitch McConnell in Kentucky, the unexpectedly strong threat to longtime Sen. Pat Roberts, R-Kansas, or the seemingly tightening contest for Georgia’s open Senate seat, most of the big money guns in the closing weeks of the campaign have been trained on three races: North Carolina, Colorado and Iowa. Those are the three lines jumping towards the top of our first chart, which shows week-by-week spending by outside groups in competitive Senate races. Hover over each line to see the race it represents and spending details.

During the past week, spending actually dropped off slightly in Kansas, where independent Greg Orman has made national headlines with his surprising show of strength against GOP veteran Roberts, and in New Hampshire, where Sen. Jeanne Shaheen is facing a challenge from Scott Brown, the former Republican senator from Massachusetts.

In the three races attracting the most outside money, a handful of powerful outside groups are driving the bulk of the spending, with party committees leading the way. While the Republican Crossroads combine, the liberal Senate Majority PAC, environmental crusader Tom Steyer’s NextGen Action Committee and the generally pro-Republican U.S. Chamber of Commerce are some of the premier forces in the races that will define these midterm elections, in the “big three” Senate races, it has been the parties leading the late charge in spending. The heavy investments in Colorado, Iowa and North Carolina from the national parties’ senatorial arms are a good indication of where the smart money is moving — and other outside players are paying attention.

Since Oct. 12 — the first week we started to notice some major separation between spending on these pivotal states and the rest of the competitive Senate races — the National Republican Senatorial Committee has led the outside spending pack with $4 million investments for Cory Gardner in Colorado and a $5 million outlay for Kay Hagan.

The DSCC is also diverting serious resources to those battleground states but its latest report to the FEC indicates the committee is borrowing money to do so..

Looking at outside spending for the most competitive Senate races, we’ve sliced and diced the data to gain insights into where the money is coming from and which givers each party is leaning on hardest. For the charts below, we’ve looked at spending both on a cumulative basis and week by week. The latter view often highlights difference in strategy — or fiscal exigencies. Here are four takeaways, drawn from a series of embeddable charts on campaign spending now available from our Real-Time Federal Campaign Finance tracker.:

1. Democrats caught up with GOP Senate outside spending in September, but Republicans now lead in total money spent.

The difference in spending is much starker when seen week by week (see below).

Republican groups outspent their Democratic counterparts by roughly $10 million for the weeks ending Oct. 19 and Oct. 26.

2. The difference isn’t just dollars, it’s who’s doing the spending — and when.

Most outside spending comes from three areas: Party committees, super PACs — which do register with the FEC and report donors — and dark money groups, which do not report their donors. Democrats have led in terms of super PAC spending overall (see below).

The week-by-week view of super PAC spending shows a sharp drop off by Republican groups late in the campaign.

When it comes to independent expenditures made by party committees, the opposite trend is visible. During the course of this election cycle, Democratic party committees have spent more on the Senate.

But the Democratic Senatorial Campaign Committee borrowed $15 million in the final two weeks of October; as of Oct. 15 the PAC owed more than $15 million and had less than $8 million in the bank. The National Republican Senatorial Committee has spent less in independent expenditures, but by Oct. 15 had about $10 million dollars in the bank with no debt.

And while weekly spending numbers are volatile, the chart suggests that DSCC’s spending seems to have slowed. That’s a reversal from September and early October, when it outspent its GOP counterpart.

3. Republican groups have dominated Senate dark money spending

For much of September and October, GOP groups–typically political nonprofits–have been outspending Democratic groups by at least $5 million a week.

4. The Democrats are keeping up in outside spending thanks to one man

With their Senate committee $7 million in debt, and dark money consistently lead by their opponents, Democrats have relied on super PACs to get their message out. After months of campaigning, few super PACs have much left to spend. The one exception? NextGen Climate Action Committee the group funded almost entirely by hedge fund billionaire Tom Steyer. The three blue peaks in the chart below largely represent infusions of his cash. Steyer has given upwards of $71 million in the last two years, leaving the group with more than $16 million to spend on Oct. 15, the most of any super PAC filing a report for that period.