One of President Barack Obama's biggest supporters will not be attending this summer's Democratic National Convention.

Sen. Claire McCaskill of Missouri told reporters Tuesday that she has never gone to conventions when she's had contested elections, and that the Washington press reaction to the news is "silly." Earlier Tuesday an aide to McCaskill confirmed to CNN that the senator will not go to the convention, which will be held in Charlotte, North Carolina during the first week of September.

"I've got a really hard election," McCaskill said. "If you had a really hard election and it was after Labor Day would you go to North Carolina to a bunch of parties and glad-handing or would you stay home and work as hard as you know how to convince Missourians they should rehire you?"

Asked if she was trying to distance herself from her party, she said "absolutely not" and explained that she's asked the president to come and campaign for her, and that Vice President Joe Biden will be in Missouri soon.

"Only in Washington would the Republican operatives get the entire press corps ginned up over the notion that I'm going to be home campaigning instead of going to a bunch of worthless parties at a convention that's only being held to do something we all know is going to happen anyway," McCaskill said.

McCaskill did not attend the 2004 Democratic convention in Boston Massachusetts. At the time she was the Democratic gubernatorial nominee in Missouri, an election she eventually lost. She did attend the 2008 convention in Denver, Colorado, when she was not on the ballot in her home state.

McCaskill endorsed then Sen. Barack Obama in January, 2008, becoming one of the first senators to back Obama, who was locked in a marathon battle with then Sen. Hillary Clinton for the Democratic presidential nomination.

McCaskill joins a growing list of Democratic senators and representatives who face tough re-elections who have said they will not attend the convention in Charlotte. West Virginia's Democratic Gov. Earl Ray Tomblin, along with the state's Democratic Sen. Joe Manchin and Rep. Nick Rahall, said earlier this month they will not attend the convention. Two Democratic New York representatives -- Rep. Kathy Hochul and Rep. Bill Owens -- also said they would not attend. Pennsylvania Rep. Mark Critz also said he would skip September's event.

On Tuesday, Montana Democratic Sen. Jon Tester and Utah Rep. Jim Matheson both confirmed they would not attend the Democratic convention. Tester's campaign spokesman said the senator would be working in Montana during the convention and has proposed a debate during that week. He refused further comment on why the Tester won't be going to Charlotte.

Matheson said his legislative duties would prevent him from attending the Democratic convention.

"I'll be spending my time this summer and fall doing my job in Congress and visiting with and listening to people in Utah," Matheson said. "I won't be traveling to North Carolina."

Congressman John Barrow of Georgia also confirmed to CNN he is not attending the convention. Barrow's decision was first reported by the Atlanta Journal Constitution. His spokesman Ricard Carbo said "it's more important for him to be in the district during that time."

Three major Republicans, businessman John Brunner, Rep. Todd Akin, and former state Treasurer Sarah Steelman, are vying to challenge McCaskill this November. The GOP Senate primary is in early August. Two of the top non-partisan political handicappers, the Cook Political Report and the Rothenberg Political Report, describe the general election battle as a toss up.

Missouri Republicans were quick to react to the McCaskill news.

"This obviously does not change the fact that McCaskill was one of Barack Obama's earliest and staunchest supporters. It doesn't erase McCaskill's enthusiastic prediction at the 2008 convention that 'Barack Obama is going to be one great president.' And it can't make up for the fact that McCaskill has voted with Barack Obama 95% of the time since 2010," said the Missouri GOP's Jonathan Prouty, in an email.

Talking Points Memo was the first to report that McCaskill would not attend the convention.