Even as President Obama has solidified his lead in recent months over Mitt Romney in Pennsylvania, Republican Senate nominee Tom Smith has been inching closer to Sen. Robert Casey (D).

(Kristen Mullen/AP)

Smith, a self-funding former Democrat who emerged from the a crowded primary field in April, has outspent Casey on the airwaves during the general election, and polling shows he stands within striking distance of the first-term Democrat with under six weeks to go until election day.

As a result of this narrowing, we are moving the Pennsylvania Senate race from “solid Democratic” to “lean Democratic” on our 2012 Senate map. The shift reflects movement toward the Republican side and suggests the race is firmly in play.

(On the map below, the yellow states are "tossup" races, the light blue and red ones are "lean" Democratic and Republican contests, and the dark blue and red states are "solid" Democratic and Republican races.) 

To be clear: Casey is still the clear frontrunner here. The Casey name is well known (and liked) in Pennsylvania, and the senator has plenty of money to spend. And with Obama looking strong in the state, Smith would likely have to run 10 points (if not more) ahead of Romney to win. That's no small feat.

That said, recent metrics suggest positive movement for Smith. A Qunnipiac University/CBS News/New York Times poll released Wednesday showed Casey leading Smith 49 percent 43 percent, a drop from the Democrat’s 55 percent to 37 percent lead in late July. A Pittsburgh Tribune-Review poll released Tuesday also showed a competitive race with Casey leading Smith 46 percent to 41 percent.

Other polls show Casey holding a wider lead. A Franklin and Marshall College poll released Wednesday showed Casey leading Smith 48 percent to 38 percent. A recent Allentown Morning Call/Muhlenberg poll showed Casey leading by 12 points, which was a drop from his previous lead of 19 points in the survey.

It’s clear that at this point, Pennsylvania more closely resembles Senate races The Fix has included in its “lean Democratic” category, including Ohio, Florida, and Michigan, than races in the “solid Democratic” group, like West Virginia, New York and Vermont.

Casey campaign manager Larry Smar told The Fix that the campaign expected the race to tighten from that 18-point spread it was at earlier this summer. He also pointed to the lopsided spending battle since the end of the primary. Smith has recently been on the air unopposed in the Philadelphia media market, and overall, been outspending Casey by a margin of about 3-1 since the last Quinnipiac poll was taken.

Casey has already gone up with attack ads against Smith, including a recent spot that hits the Republican for saying he started a tea party group. Look for Democrats to continue trying to cast Smith as too conservative for the state.

Smith’s campaign says it has been focused on disproving the notion that Casey is a moderate. A recent campaign ad casts Casey as a tax-raiser and hits him for voting for Obama’s health care law.

At this point, Pennsylvania is not among the closest Senate races across the country. But it’s well worth keeping an eye on. If it catches the attention of the national senatorial committees enough to spur the groups to spend money on ads down the stretch, it will be a sign the race has tightened even more.