Less than a month before Election Day, with voters already beginning to head to the polls in many states, outside spending to influence campaigns up and down the ballot is on a consistent upward trend, topping the $100 million mark in one week for the first time.

Since Sept. 7, when all independent political expenditures — no matter what their content — began to be reported to the Federal Election Commission, about $360 million has been dropped by outside spending groups on Senate and House races as well as the presidential contest. Republican-affiliated outside spending groups have outpaced their Democratic counterparts by some $56 million, an analysis of FEC data compiled on Sunlight's Follow the Unlimited Money has found.

And while much of the media focus this year has been on the impact the independent expenditures are having on the presidential race, Sunlight's analysis shows that the big spenders are beginning to give equal attention to the contests that will determine control of the House and Senate next year.

House and Senate races each drew more spending than the presidential race last week. In the electioneering period, House races have drawn $130.4 million, compared to $128.8 million in the contest for the Oval Office. Senate races attracted $102.5 million. 

The weekly total of outside spending has skyrocketed from $26.2 million in the second week of September to more than $102.3 million for the week that ended Thursday. The 60-day window that began Sept. 7 is significant because committees are required to disclose all spending they make to influence elections during this period, even if it is "electioneering," communications that don't directly urge votes for or against named candidates. Except for the first week in the 60-day window, spending by outside groups, such as super PACs, 501(c)4 non-profits and party committees, has favored Republicans over Democrats by at least $7.5 million. 

In the last week — when the gap in spending between Republican and Democratic groups has narrowed — notable expenditures include:

• Republican-leaning non-profit Crossroads GPS and its super PAC arm, American Crossroads, spent a combined $16.8 million. Roughly half went to influence tight Senate races in Virginia, Florida, Ohio, Wisconsin, Massachusetts, Montana, North Dakota, Indiana and Nevada. 
• The Democratic Senatorial Campaign Committee and the Democratic Congressional Campaign Committee dropped about $16.3 million combined. DSCC targeted many of the same states as Crossroads, as well as in Senate contests in Arizona, Connecticut, Maine and Missouri. All of the DSCC spending went for negative campaigning — to oppose Republicans in those races. 
• Unions have also stepped in to boost Democratic spending. The Service Employees International Union super PAC and its non-party committee affiliate SEIU Committee on Political Education spent over $4.6 million across the country, with a large $1.1 million to support President Barack Obama. The American Federation of State, County and Municipal Employees also spent about $1.8 million in favor of Democrats. 
• The National Rifle Association dropped $3.9 million in the last week, of its total $6.1 million in spending since February last year.