Sen. Claire McCaskillClaire McCaskillOversight panel demands answers on Pentagon waste report Messer eyes challenging Donnelly for Indiana Senate seat Overnight Defense: Funding bill would ease Trump Defense pick's confirmation | Obama delivers final security speech MORE (D-Mo.), one of President Obama's biggest surrogates in 2008, will skip this summer's Democratic National Convention.

The announcement makes McCaskill, who faces a tough reelection bid, the latest red-state Democrat to announce she won't be in Charlotte, N.C., in early September.

"Generally speaking, Claire has not gone to the national convention when she is on the ballot because she believes it's important to spend as much time as possible in the state of Missouri talking with voters," said a McCaskill campaign aide.

The aide points out that McCaskill also did not attend the convention in 2004, when she was running for governor, although she did attend in 2008 — and spoke at the convention.

The one-term senator has had to carefully calibrate how closely she allies herself with Obama. She is a top GOP target and running in a Republican-trending state.

Missouri Republicans are working hard to emphasize her ties to the president.

“No matter how desperately Claire McCaskill tries to distance herself from Barack ObamaBarack ObamaBrian Williams slams fake news Obama: I absolutely faced racism while in office Unfinished business: Who will speak for the women of the world now? MORE, it just won’t work.  Skipping her party’s convention will not make Missourians forget about McCaskill’s rubberstamp support for Barack Obama’s reckless agenda on everything from the failed stimulus to Obamacare to devastating EPA regulations to record spending and debt," said Lloyd Smith, executive director of the Missouri Republican Party, in a statement.

The Hill rates the race a "toss-up."

Talking Points Memo first reported that she'd skip the convention.