National Democrats have more riding on Pennsylvania’s Senate race between former Rep. Pat Toomey (R) and Rep. Joe Sestak (D) than any other contest this cycle. Literally.

The Democratic Senatorial Campaign Committee has invested more than $4.7 million in the Keystone State to date, not including the nearly $1.2 million that it spent on Sen. Arlen Specter’s failed primary bid.

The overall investment, expected to exceed $9 million by Election Day, is more than any other state in the nation. And while the numbers will change dramatically in the coming weeks, the Pennsylvania commitment to date dwarfs the Democratic committee’s efforts in other competitive races such as Illinois ($2.66 million), Missouri ($2.36 million), West Virginia ($1.87 million), Connecticut ($1.09 million), Kentucky ($647,000), Nevada ($53,000) and New Hampshire ($26,000), according to Federal Election Commission filings. Only the $5.3 million that the DSCC has spent in Colorado is comparable.

The DSCC’s Pennsylvania investment also dwarfs that of its Republican counterpart.

Aided by a flood of independent spending from conservative groups, the National Republican Senatorial Committee has spent less than $630,000 in Pennsylvania so far and waited two months longer than the DSCC to run its first television spot there. While the NRSC disclosed plans to spend a total of $3.7 million in the coming weeks, doubling its previous reserve, that’s still less than half of what Democrats are expected to spend by Nov. 2.

Republicans suggest the investment hasn’t worked.

“There are so many things that we have going for us in this race,” Toomey spokeswoman Nachama Soloveichik said. “I think that despite what people want to say about internal polls, the public polls have shown we have an advantage in this race.”

Indeed, while there are conflicting polling numbers circulating, Toomey is ahead in virtually every head-to-head matchup, aside from a DSCC internal poll released this week that showed Sestak with a statistically insignificant 2-point edge.

Republicans concede their internal polling shows Sestak trailing by from 4 to 6 points, but he was behind by 10 points in a Rasmussen Reports poll released Thursday.

“This isn’t investing against all odds. This is investing in a state where Democrats have had success in the last few cycles, where they have a 1.2 million-person registration advantage,” said Pennsylvania-based Democratic strategist Mark Nevins, a partner at the Dover Group.

Democrats pounced on Wednesday’s news that the NRSC launched its first independent television buy in Pennsylvania — $618,000 for a 30-second spot that banged Sestak for being too far to the left. Republicans note, of course, that the DSCC have been spending on television in Pennsylvania since Aug. 13. The NRSC waited until Oct. 13 — two months later — to make its first independent expenditure.

That’s not to say that it has ignored the race before this week. The NRSC has given Toomey’s campaign the maximum coordinated donation: $1.7 million.

And putting the campaign committees aside, few places showcase the disparity between conservative and liberal independent spending better than Pennsylvania’s Senate contest.

Conservative groups — led by Toomey’s former employer, the Club for Growth — have spent roughly $4.5 million to date. Other groups spending heavily on the race include the U.S. Chamber of Commerce and American Crossroads.

Democratic-friendly groups — including the League of Conservation Voters, the National Wildlife Federation and VoteVets.org — have spent less than $600,000 combined so far.

The disparity puts serious pressure on the DSCC to pick up the slack, Nevins said.

“If they’re going to keep Sestak going and build a firewall here in Pennsylvania, then it’s incumbent upon them to invest here,” he said.

The outside groups are allowed to coordinate spending strategy with each other, but not with the campaign committees or candidates. Asked about the spending disparity between the party committees in Pennsylvania, NRSC spokesman Brian Walsh said there was no coordination with the conservative groups.