Advocates for tougher gun laws are planning to flex their muscles in this fall's critical Senate elections -- but they have a dilemma in Pennsylvania.

While the Democratic challenger, Katie McGinty, favors the full lineup of ideas backed by gun control groups, the incumbent Republican, Pat Toomey, has also won praise as one of the few in his party willing to support broader background checks.

The quandary was reflected Tuesday in a rally and press conference in which those pushing for tougher gun laws promised to make guns a critical issue in key races this fall. At the Logan Square rally featuring former Arizona Congresswoman Gabby Giffords and CeaseFirePA, former Gov. Ed Rendell chided Toomey for not doing enough on guns after his 2013 background check bill failed. He also criticized Toomey's recent votes against Democratic plans to bar terrorism suspects from buying guns.

"He went south on us," said Rendell, who is McGinty's campaign chairman. "We have to send Pat Toomey a message, that lip service is not good enough -- we want action."

But shortly later, in a meeting with reporters in a Marriott conference room, Giffords' husband, Mark Kelly, praised Toomey, along with McGinty.

"To have two people that are on the same side of this issue, it is in the best interest of communities here in Pennsylvania and around the country," Kelly said.

While the group he and Giffords founded, Americans for Responsible Solutions is already on the air hammering Republican Sen. Kelly Ayotte in New Hampshire, Kelly said they had not decided if they would get involved in Pennsylvania's senate contest, one of the toughest in the country and a top priority for Democrats.

Kelly said they disagreed with Toomey's recent votes on barring terror suspects from purchasing guns, but added, "you've got to give him a lot of credit" for his background check bill with Sen. Joe Manchin (D., W.Va.), and trying to convince other Republicans to support it.

"We worked with him on this and worked with his office and we think he did a reasonable job with what he had to work with," Kelly said.

Sitting next to him at the press conference, though, Sen. Chris Murphy (D., Conn.) jumped in -- arguing that there is a difference between voters choosing between two options, and a political group deciding where to spend limited resources.

"If you are a voter in Pennsylvania who cares about ending gun violence, Katie McGinty is going to be a better advocate and voice for you," Murphy said. "But from the movement's perspective, you have to set priorities."

Murphy has become one of Congress' leading voices pushing for tougher gun laws since the Newtown school massacre in his state, and recently led a 15-hour filibuster on the senate floor. Democrats' hopes of taking control of the Senate may depend on unseating Toomey -- and they have attacked him for not doing enough on guns since his 2013 bill failed.

The two events Tuesday came hours before the Democratic National Convention plans to feature on its main stage mothers who have lost children to shootings. Kelly said the gun debate's major role in the convention shows that the politics of the issue have shifted -- that the public supports tougher gun laws and that it can be a winning campaign issue, not the political loser many have long feared. He and Giffords plan to tour the country this fall, he said, injecting the issue into close races.

"We must never stop fighting," Giffords said at the Logan Square rally. The former Congresswoman was badly wounded in a mass shooting in her home state in 2011. "Fight, fight, fight."

The challenge for gun control advocates, however, is matching the intensity of those who say new gun laws infringe on Second Amendment rights. Rendell said people who want tougher gun laws have to become "single issue" voters, much like those on the other side.

Toomey's campaign said he will "continue his leadership on finding common ground to fight gun violence, because the only way to accomplish real reforms is to work together across the aisle, and not just talk past one another."

McGinty's spokesman, Sean Coit, responded that "the idea that Pat Toomey is a moderate on guns is a lie, plain and simple." He pointed to recent Toomey comments telling voters that he has "a perfect track record" with the National Rifle Association.

Toomey favors expanding background checks -- an idea the NRA has worked to block -- but he opposes bans on so-called assault weapons and high-capacity magazines. He recently voted against bills, favored by gun control advocates, that would bar anyone on terror watch lists from buying firearms, saying the Democratic measures did not include enough due process for innocent gun buyers. He instead supported weaker Republican version, and a narrower compromise proposal.

McGinty favors expanding background checks, barring assault-style weapons and the Democratic plan on terror suspects.

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