Republican incumbents are leading their Democratic Senate challengers in four crucial swing states, according to a Quinnipiac University poll released Friday.

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GOP Sens. Marco Rubio Marco Antonio RubioMurky TikTok deal raises questions about China's role Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Florida senators pushing to keep Daylight Savings Time during pandemic MORE (Fla.), Richard Burr Richard Mauze BurrRep. Mark Walker says he's been contacted about Liberty University vacancy Overnight Defense: Trump rejects major cut to military health care | Senate report says Trump campaign's Russia contacts posed 'grave' threat Senate report describes closer ties between 2016 Trump campaign, Russia MORE (N.C.), Rob Portman Robert (Rob) Jones PortmanMcConnell locks down key GOP votes in Supreme Court fight Romney undecided on authorizing subpoenas for GOP Obama-era probes Congress needs to prioritize government digital service delivery MORE (Ohio) and Pat Toomey (Pa.) all lead their opponents, in addition to outperforming the Republican at the top of the ticket, Donald Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE.

Toomey holds the narrowest lead, outpacing Democratic candidate Katie McGinty by just 1 point, 46 percent to 45 percent.

The GOP’s strongest race of the four is in Ohio, where Portman is leading former Gov. Ted Strickland by 11 points, 51 percent to 40 percent.

Strickland’s bid is complicated by the fact that he is struggling to court women voters, who traditionally lean Democratic. Forty-six percent of female likely voters in Ohio are backing Portman, while 45 percent are supporting Strickland.

And in North Carolina, Burr is leading Deborah Ross 49 percent to 43 percent.

Rubio holds a similar lead over Florida Rep. Patrick Murphy (D), outpacing him by 7 points, 50 percent to 43 percent.

The poll surveyed 601 Florida likely voters with a margin of error of 4 percentage points, 751 North Carolina likely voters with a margin of error of 3.6 percentage points, 775 Ohio likely voters with a margin of error of 3.5 percentage points and 778 Pennsylvania likely voters with a margin of error of 3.5 percentage points.

A separate Quinnipiac survey released Thursday showed Democratic presidential nominee Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE leading Trump in both Pennsylvania and North Carolina, tied with him in Florida and trailing him by a single point in Ohio. Taken together, the polls indicate many swing state voters may be ready to split their ticket.