The survey, conducted by OnMessage Inc. for the National Republican Congressional Committee, gives Barber 46 percent support to McSally's 45 percent support — within the poll's 4.9-point margin of error.

But Barber has kept up a steady fundraising pace to prepare for what could be a difficult reelection fight. His campaign said the candidate raised $293,000 in the second quarter, about the same amount he raised in the first, and had $550,000 cash on hand at the end of June.

It's an indication, pollster Wes Anderson writes, that Barber is "one of, if not the most vulnerable" Democratic House incumbents this cycle.

Though McSally hasn't yet announced a run, she filed papers last week, setting up a rematch of one of the nation's closest races of the 2012 cycle.

Barber won by less than 2,000 points, and the district leans slightly Republican — the GOP has won it on the presidential level during the past two cycles.

Barber's campaign spokesman Rodd McLeod told the Arizona Republic that the congressman wasn't concerned about his campaign at this point in the cycle.

Ron BarberRon BarberGiffords to lawmakers avoiding town halls: 'Have some courage' Ten House seats Dems hope Trump will tilt House conducts moment of silence for Tucson shooting anniversary MORE is working hard to rebuild southern Arizona’s middle class by looking out for our veterans and small business. And he is focused on improving Arizona’s economy and border security,” he said. “We’ll worry about a political campaign next year.”

OnMessage surveyed 400 likely 2nd District voters from June 17-18 for the poll.

--This piece was updated at 3:42 p.m. to reflect Barber's fundraising numbers.