WASHINGTON – A key Democratic Super PAC is stepping into the Pennsylvania Senate race, launching a series of digital ads attacking incumbent Republican Sen. Pat Toomey.

The ads, to appear on Twitter, Facebook and in Google search results for about a week, paint Toomey as a lock-step Republican, citing his frequent votes – more than 90 percent – with GOP leadership and his perfect rating this year from Americans for Prosperity, a conservative group backed by the Koch brothers.

"The Republican agenda puts special interests and partisan ideology ahead of what's best for everyday Americans and Pat Toomey is the poster boy for those misplaced priorities," said Shripal Shah, a spokesman for Senate Majority PAC, which backs Democrats in key swing races.

The ads are the first in a $1.5 million national campaign from the group, which also plans to target other swing state incumbents. They come roughly two weeks after Toomey launched his own attack site, "WhoIsWorse.com" assailing Democratic contenders Joe Sestak and Katie McGinty.

The site labels Sestak an extreme liberal while accusing McGinty of profiting while she moved between the public and private sectors.

Together, the online mud-slinging gives an early preview of the attacks lines expected to come into focus next year as Toomey and the Democrats try to define one another.

Toomey, seeking re-election in a state that tilts blue in presidential election years, has presented himself to voters as a conservative who is willing to cross party lines -- a message Democrats hope to undercut. The Lugar Center, affiliated with Georgetown, rated Toomey as the 40th most bipartisan Senator in the previous Congress, and Toomey's campaign pointed to bipartisan bills to expand background checks for guns and to prevent child predators from getting jobs in schools.

"Pat is proud of his work across party lines to make our communities safter and our economy stronger," said Toomey spokesman Steve Kelly. "Suggestions to the contrary are nothing but election season political attacks."

Sestak, McGinty and Braddock Mayor John Fetterman are running in the Democratic primary to challenge Toomey, who is completing his first term. Pennsylvania's race is expected to be one of the hottest in the country, and could help decide control of the Senate.

You can follow Tamari on Twitter or email him at jtamari@phillynews.com.