WASHINGTON — One of US President Barack Obama's top backers in Congress, Senator Claire McCaskill, will skip September's Democratic convention as she focuses on her own tough re-election battle, her office said Tuesday.

"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," a McCaskill aide said in a statement.

It's an all-too-common rationale for a lawmaker skipping a glitzy political confab far from her constituents, but it resonates for McCaskill, who campaigned vigorously for Obama in 2008, as she is now seeking a second term in a state that is leaning Republican.

McCaskill skipped the 2004 Democratic convention when she was running for governor, a race she lost.

Her latest decision can't be seen as helpful for Obama, who is locked in his own re-election race against Republican Mitt Romney.

Obama narrowly lost Missouri to Republican candidate John McCain in 2008, and trails Romney in recent polls.

Three leading Democrats from West Virginia, including Governor Earl Ray Tomblin and Senator Joe Manchin, have also said they will skip the convention, according to The New York Times.

All three are up for re-election in November in a state where Obama is deeply unpopular. In the Democratic presidential primary last month, a federal prison inmate running as a protest candidate won 40 percent of the vote against Obama.

Obama's poor showing in West Virginia is partly due to his efforts to promote alternative energy in the coal-rich state.

The Democratic National Convention, held in early September in Charlotte, North Carolina, will see Obama officially accept his party's nomination and serves as a kickoff for the final campaign push ahead of the election.

The Republicans hold their national convention days earlier, in Tampa, Florida.