(CNN) - Mitt Romney hopes this November to do what no Republican has done in 24 years: win Pennsylvania in a presidential election.
The presumptive GOP nominee returns to the Keystone State on Tuesday, holding a campaign rally in the Pittsburgh suburb of Irwin.
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Vice President George H.W. Bush was the last Republican to win Pennsylvania, taking the state over Massachusetts Gov. Michael Dukakis in the 1988 election. The state was a battleground in the following presidential cycles, but four years ago then-Sen. Barack Obama topped Sen. John McCain by 10 points in Pennsylvania. But Republicans stormed back in the 2010 midterm elections, capturing the governor's office, a U.S. Senate seat, and five House seats from the Democrats.
A non-partisan poll conducted last month by Quinnipiac University showed Obama with a 45%-39% advantage over Romney, with one in 10 registered voters in Pennsylvania undecided.
"Make no mistake that Pennsylvania is in play," a Romney campaign official tells CNN, adding that "it is one of the most productive states we have in terms of volunteer voter contact."
The Romney campaign says it has 21 field offices up and running in the state. The Obama campaign says it has 30 offices in the state.
Romney was last in the state in mid-June, taking a bus tour through Weatherly, Quakertown, and Cornwall in eastern Pennsylvania.
Irwin, site of Tuesday's event, is in Westmoreland County, which McCain handily won in 2008. The county, along with the other Pittsburgh suburbs, could play an important role in determining who wins the Keystone State's 20 electoral votes, along with the Lehigh Valley and especially the swing suburbs (Montgomery, Delaware, Chester, and Bucks) of Philadelphia.
A million more people identify themselves as Democrats than Republicans (4.14 million-3.08 million), according to the latest figures from the Pennsylvania Department of State, and just over a million votersare non-affiliated or identify with third-parties.
The president last campaigned in the state two weeks ago, holding a rally in Pittsburgh. Vice President Joe Biden, who was born and grew up in the Keystone State, was last there on July 2.
The Obama re-election campaign has spent slightly more than $3.5 million to run television commercials in Pennsylvania, according to data provided to their clients by Kantar Media/Campaign Media Analysis Group, which tracks political ad spending on broadcast and national cable TV.
The data covers the period from April 10 through July 15. April 10 is the day former Sen. Rick Santorum of Pennsylvania suspended his presidential campaign. Santorum was Romney's main rival for the nomination after Newt Gingrich's star faded and Romney became the presumptive GOP nominee when Santorum dropped out.
According to CMAG, Romney has yet to run general election ads in the state. But Restore Our Future, the pro-Romney super PAC, has shelled out nearly $1.7 million to run ads. Crossroads GPS, a pro-Republican advocacy group, has spent nearly $1 million to air spots, and Americans for Prosperity, another independent group supporting the GOP, has spent more than $2 million to run commercials. A pro-Obama advocacy group has spent just more than $1 million to run commercials in Pennsylvania.
"There's been an interesting split between Democratic and Republican ad activity. Two well-resourced GOP groups, Americans for Prosperity and Restore Our Future, have invested in advertising in the expensive Philadelphia market; Democrats for the most part have not," CMAG Vice President Elizabeth Wilner told CNN. "Democrats have focused their advertising on western Pennsylvania."
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