Pennsylvania Senate race now the most expensive ever as outside money pours in

Sen. Pat Toomey, R-Pa.
Tom Williams | CQ Roll Call | Getty Images

Outside groups have funneled cash into key Senate races this year as the major parties fight for control, and one contest has already become the most expensive ever for a Senate seat.

Candidate committees and independent groups have spent more than $113 million on the Senate race in the key state of Pennsylvania, where incumbent Republican Pat Toomey is fighting off a challenge from Democrat Katie McGinty, according to Federal Election Commission data compiled by the Center for Responsive Politics. With two weeks to go, it has already surpassed the previous most expensive race, the 2014 North Carolina contest won by Sen. Thom Tillis.

Tight Senate races outside of Pennsylvania are also drawing huge money. More than $95 million has already been spent in the New Hampshire contest pitting incumbent Kelly Ayotte against the state's Gov. Maggie Hassan. At least $83 million has been spent in the Nevada race featuring Republican Joe Heck and Democrat Catherine Cortez Masto.

The spending in these key race underscores the growing power of independent groups and political action committees as the Republicans and Democrats fight for control of the Senate. Some of those groups are not subject to the contribution limits facing campaign committees.

The vast majority of spending in the 10 most expensive Senate races has come from outside groups. Eight of those races feature Republican incumbents aiming to hold on to their seats.

Many outside organizations have mobilized in the Senate fight as Republicans losing control of the Senate has become more of a reality amid GOP presidential nominee Donald Trump's recent struggles. For instance, a pro-Hillary Clinton super PAC, Priorities USA Action, has started to launch attacks on GOP Senate candidates like Ayotte.

In Pennsylvania, an average of recent polls shows first-term senator Toomey with a nearly 2-point lead over McGinty, a former environmental policy official. Clinton has a nearly 6-point lead over Trump in recent surveys of the state.

Senate candidate from Pennsylvania Katie McGinty.
Bill Clark | CQ Roll Call | Getty Images

Hassan holds a slight edge in New Hampshire in recent surveys, while Cortez Masto has a roughly 2-point lead in Nevada.

Republican Sen. Roy Blunt holds a narrow lead in recent Missouri surveys, while Sens. Richard Burr, Marco Rubio and Rob Portman have more comfortable advantages in North Carolina, Florida and Ohio, respectively. Meanwhile, Democrats Tammy Duckworth and Russ Feingold have comfortable cushions in recent surveys of their races against incumbents in Illinois and Wisconsin, respectively.