Sen. Pat Toomey (R., Pa.) refused to rule out supporting Donald Trump for president Tuesday, despite mounting political pressure and high-profile defections from other GOP lawmakers.

Toomey said definitively he would not vote for Democrat Hillary Clinton -- calling her "one of the most flawed nominees in the history of the republic," -- but left open the possibility that he might eventually support Trump, his party's nominee.

Locked in one of the country's toughest Senate races, Toomey has not endorsed Trump and reiterated Tuesday that he has "serious reservations, serious concerns," about the New Yorker. But when asked if he could rule out backing Trump, Toomey said, "I remain unpersuaded."

He spoke Tuesday morning in Villanova at a "Women for Toomey" campaign stop where he was joined by a few dozen female supporters and Sen. Susan Collins (R., Me.) -- who in August said she could not support Trump. Collins stood alongside Toomey as he answered questions from reporters, but did not weigh in on the presidential race.

Toomey's comments marked the first time he has taken reporters' questions since the Friday release of a 2005 video capturing Trump bragging in lewd terms about aggressively kissing and groping women.

He has condemned Trump's words, but asked if that video had failed to persuade him to make a decision -- with four weeks to go until Election Day -- Toomey said "I had hoped that Donald Trump would persuade me to be an enthusiastic supporter," but that he has not done so.

Since the recording's release Trump's poll numbers have plunged and a number of prominent Republicans, including several fellow senators and Pennsylvania Congressman Pat Meehan and Charlie Dent have definitively said they will not support Trump for president. Toomey declined to take that step.

"I made it clear way back in May, way back when Donald Trump emerged as the likely nominee, that I had serious reservations, serious concerns, I still do," Toomey said Tuesday. "I will continue to speak out when I think it's appropriate."

He was more definitive on Clinton: "I think Hillary Clinton is a hopelessly flawed candidate, I could never support Hillary Clinton."

While Trump's comments have threatened to make him toxic with moderate voters and wipe out fellow Republicans, abandoning Trump also comes with the risk of offending his fervent supporters on the right.

Toomey blasted his Democratic challenger, Katie McGinty, for her support of Clinton, saying, "that kind of reflexive partisanship is not helpful in Washington, it's not going to help us break the gridlock that we have."

McGinty, in downtown Philadelphia two hours after Toomey's event, said the senator's refusal to disavow Trump ruins the incumbent's credibility.

"Either Sen. Toomey does feel that the kind of disgusting comments and behavior we've seen from Donald Trump are absolutely all right ... or Sen. Toomey is not telling us the truth and he knows, as we know, that Donald Trump is 100 percent totally unfit to be president of the United States… That's craven politics. It's disqualifying," McGinty said.

She was energized as she skewered Trump and Toomey, standing with about a dozen women on John F. Kennedy Boulevard in front of the building that houses Toomey's Philadelphia office.

Toomey has painted McGinty as a "rubber stamp" for Clinton while touting his own independence and willingness to stand up to his party. McGinty said the senator should prove it.

"How about standing up to Donald Trump right here in Pennsylvania today?" McGinty asked. "How about calling for an investigation of Donald Trump into his confessed illegal behavior?"

Polls have shown a tight race in a contest expected to help determine control of the Senate.

Staff writer Justine McDaniel contributed to this report.

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