Look no further for evidence of a fluid Senate landscape than where the party committees and outside groups have spent the most money over the past week.

According to data compiled by the Campaign Finance Institute, a Washington, D.C.-based nonpartisan think tank, almost $29 million in independent expenditures was spent on Senate races in the 10 most-targeted states during the past seven days. The presidential battleground states of Ohio and Virginia top the list, but they’re followed by none other than Indiana and Arizona — two states with GOP-held open seats that were among the last to emerge onto the competitive playing field.

Also included in that group is Nevada — a potential Democratic pickup that SKDKnickerbocker Managing Director J.B. Poersch believes, along with Maine and Massachusetts, give Democrats the edge in their quest to maintain control of the Senate.

“Several Senate races will go to the wire,” said Poersch, a strategist for the Democratic-aligned Majority PAC and former executive director of the Democratic Senatorial Campaign Committee. “But, given that Democrats have a legitimate chance to win in five GOP-held states, it’s more than likely we hold the majority.”

The path for Republicans to wipe out the Democrats’ 53-47 majority may have narrowed during the past few months with so many close races and the uncertainty surrounding GOP Rep. Todd Akin’s challenge of Sen. Claire McCaskill (D) in Missouri.

Yet the fight goes on, as every day another outside political group is launching a new round of television advertising in one of the contested states.

The emergence of the open seat in Connecticut as a competitive race has given Republicans another offensive opportunity that wasn’t necessarily apparent a year ago, and even the re-election race of Sen. Bob Casey (D-Pa.) is far closer than ever imagined.

By the beginning of this week, national party committees and outside groups had spent more than $180 million on IEs in Senate races for the cycle, according to the numbers compiled by the CFI. With two weeks remaining and Senate control at stake, the IEs had already surpassed that of the entire 2010 cycle.

Much of the Senate spending that hasn’t flowed from the DSCC or the National Republican Senatorial Committee has come from a few groups with the goal to win the Senate majority. Just Tuesday morning, the Majority PAC released spots in Montana and Virginia, as the Republican-aligned Crossroads GPS and its affiliate American Crossroads launched ads there and six other states.

“Crossroads will keep ratcheting up the pressure to stop the wasteful spending, massive tax hikes and government health care takeover that these liberal politicians support,” Crossroads GPS spokesman Nate Hodson said in a statement accompanying one of the ad buy announcements.

The shifts in the competitive playing field include an unlikely amount of offensive opportunities for Democrats, who are defending more than twice as many seats as Republicans. Rep. Joe Donnelly (D), who is running for the open Republican-held seat in Indiana, and former Surgeon General Richard Carmona (D), running for the open GOP-held seat in Arizona, released polls today that showed them with small leads.

The CFI list of states with the most outside spending in the past seven days is topped by Ohio, where about $25 million in independent expenditures has been spent so far, including $5.4 million in the past week. Despite the heavy spending in the state, which could decide the presidential race, a Quinnipiac University survey released Monday found Sen. Sherrod Brown (D) with a comfortable 9-point lead over state Treasurer Josh Mandel (R). However, the spending there indicates neither side believes the race is over.

About $3.8 million was spent in the past seven days in Virginia, another top presidential battleground, where former Govs. Tim Kaine (D) and George Allen (R) both have reason to be confident heading into the final days. The Old Dominion has seen the most spending overall, with about $30 million in IEs spent so far.

Beyond the top four of Ohio, Virginia, Indiana and Arizona, the states that saw the most outside IEs for Senate races in the past week were: Wisconsin, Montana, Missouri, Connecticut, Nevada and North Dakota.

All but Nevada are offensive opportunities for Republicans. Massachusetts, where Sen. Scott Brown (R) is being challenged by Harvard University professor Elizabeth Warren (D), would likely have topped this list if the two hadn’t agreed on a pledge that has successfully kept outside spending to a minimum.

Spending has slowed considerably in the final stretch in prohibitively expensive Florida, where Sen. Bill Nelson (D) is favored for re-election against Rep. Connie Mack IV (R). Early spending against Nelson has kept Florida among the top six targets for outside IE spending for the entire cycle at more than $14 million, but only $549,000 was spent in the Sunshine State in the past week.

That’s good for only the 13th highest, according to the CFI list.

Correction: 12:50 p.m.

An earlier version of this article misidentified one of the states that J.B. Poersch listed as possible Democratic pickups. He mentioned Massachusetts, as well as Maine, Nevada, Arizona and Indiana.