WASHINGTON (AP) -- Democrats are sounding increasingly concerned about their chances of retaking control of the Senate, as Republicans demonstrate a commanding fundraising advantage and Hillary Clinton's lead narrows in key battleground races.

Although most Democrats still express confidence that they will win back the Senate majority in November, they now appear to have fewer paths to victory as wins in Ohio and even Florida look increasingly remote.

And if they do win back control, it could end up being with the narrowest of margins, even a 50-50 Senate with a Vice President Tim Kaine casting tie-breaking votes for the Democrats, if Hillary Clinton becomes president.

A key factor is the Republican money edge, which is particularly pronounced this year because some major donors, most notably the billionaire Koch Brothers, have decided to stay out of the presidential race out of distaste for Donald Trump and are pouring money into Senate races instead. Ohio, Florida, Nevada and other races are awash with cash.

"It's worrisome," the No. 2 Senate Democrat, Dick Durbin of Illinois, said Tuesday of the GOP money advantage. Overall, Durbin offered "mixed reviews" of the Senate map: "Solid, quality candidates, good campaigns but a massive infusion of Republican money in the last few weeks, and we are working overtime to try to keep up with it."

Democrats frequently point with alarm to the massive $42 million haul in August disclosed by two connected fundraising committees run by allies of Senate Majority Leader Mitch McConnell, R-Ky. That money is now being funneled to New Hampshire, Nevada, Indiana and elsewhere.

Senate Democrats have been pressed to chip in more to make up the deficit. Last week, Sen. Jon Tester of Montana, who heads the Senate Democratic fundraising arm, announced in a private meeting that Sen. Chuck Schumer of New York had transferred $2 million from his campaign accounts to the Democratic Senatorial Campaign Committee.

Another concern is Clinton's sometimes weak performance as a candidate, particularly pronounced over the past days as her campaign contended with questions over its handling of her health. Clinton's stumbling exit from a 9/11 memorial Sunday was followed by the disclosure that she had been diagnosed with pneumonia.

Former Pennsylvania Gov. Ed Rendell said he was "surprised and a little frustrated" that the race was so close, and said it was a concern as Democrats seek to recapture the Senate.

"In Pennsylvania for example, if Hillary does win by the 7 she's up in the latest poll it will be very hard for (incumbent Republican Pat) Toomey to win," Rendell said. "If Hillary wins by 2 or 3, it gives Toomey a chance."

Republicans now have a 54-46 majority in the Senate, so Democrats have to net at least four seats to take back the majority, or five if Donald Trump becomes president, because of the vice president's role as the Senate tie-breaker.

Democrats entered this election cycle with some expressing high hopes because of a favorable Senate map that has Republicans defending 24 seats and Democrats only 10. Republicans have vulnerable incumbents in blue or purple states including Illinois, Wisconsin, New Hampshire, and Pennsylvania, while Democrats are defending only one at-risk seat, in Nevada, where Senate Minority Leader Harry Reid is retiring.

Partisans on both sides are acutely aware that the map reverses itself in 2018 when Democrats will be playing defense on GOP-friendly terrain such as Missouri, North Dakota, Montana and West Virginia. That makes the task for Democrats all the more urgent this year since they will be at greater risk of losing seats two years from now.

While some predicted that Trump at the top of the ticket would pull down Republican incumbents from coast to coast, that hasn't happened in all cases. Instead, lawmakers including Rob Portman in Ohio, Marco Rubio in Florida and Toomey are running ahead of Trump in their states, and either beating their opponents or keeping it close.

"All I can say is that I was feeling much better a few weeks about the chances of Democrats retaking the Senate," said Jim Manley, a Democratic consultant and former top Reid aide. "I still think it's going to happen, because the Trump campaign is going to prove toxic to the Republican Party, but it may prove to be tougher than many Democrats were thinking a few months ago."

Officials with the Democratic Senatorial Campaign Committee dispute suggestions that the outlook has changed for the worse, noting that unlike some others, they never predicted they would sweep to victories across the country and end up with a comfortable majority.

While disagreeing that they are giving up on Florida and Ohio, where they have delayed spending, DSCC officials point to plans to spend in North Carolina and Missouri. It's cheaper in those states to pay for advertising, though Missouri is widely considered a long shot.

Tester said he would "of course" feel more confident if Clinton had a bigger lead, but added: "The truth is that Hillary's running a great race, I'm confident that she's going to win, but nothing's easy. I mean these are hotly contested races and we knew that 20 months ago, and that hasn't changed."

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Associated Press reporter Catherine Lucey contributed to this report.

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