By Bruce Olson

ST. LOUIS (Reuters) - Missouri Democratic Senator Claire McCaskill was flying coach on a commercial flight to St. Louis on Thursday after days of battering from Republicans over her failure to pay taxes on a private jet she owns.

The senator has already paid nearly $300,000 in back property taxes on the plane and owes more for interest and penalties.

McCaskill is up for reelection in 2012 and is facing an energized Republican Party, which holds majorities of both houses of the Missouri legislature and needs a gain of just four seats to take over control of the U.S. Senate.

The flap over the private airplane, which some in the Missouri media are calling "Air Claire," has prompted at least one respected political newsletter to downgrade her prospects for next year.

The Cook Political Report had Missouri in the "lean Democratic" column in the 2012 Senate race until the controversy over the plane.

"Over the past two weeks, McCaskill's road to reelection has hit some turbulence largely of her own making that has only served to render the already vulnerable incumbent even more vulnerable," the report said.

"McCaskill's path to reelection has become bumpy enough to warrant a rating change from Lean Democratic to Toss Up," the report added.

McCaskill purchased the single-engine, turbo-prop Pilatus in 2006 and parked it at the Spirit of St. Louis Airport in suburban St. Louis. Since the plane was housed in Missouri, McCaskill was required to pay personal property tax on the $2.1 million plane.

Her personal worth, according to the Center for Responsive Politics, is somewhere between $15 million to $27 million, due largely to the fortune of her husband, who made his money in the affordable housing industry.

She faces a demand from the Missouri Republican Party that the Senate Ethics Committee open hearings on possible improper use of the plane.

She said she overlooked the taxes until earlier this month and on Monday wrote checks for what she thought she owed. She said at the time she realized additional payments would be necessary and was awaiting a bill from the county.

She also said she was getting rid of the plane and in future would fly coach.

The senator earlier this year reimbursed the federal government $88,000 to cover the taxpayer-paid cost of 90 trips she took in the plane, at least one of which she said was for a political event.

Two Republicans have declared their intention to run for the nomination to face McCaskill next year: St. Louis lawyer Ed Martin, who lost a bid for Congress last year, and former state Treasurer Sarah Steelman.

(Editing by Greg McCune)