Sen. Claire McCaskillClaire Conner McCaskillThe Hill's Morning Report — Can the economy help Republicans buck political history in 2018? Dems seek to leverage ObamaCare fight for midterms Dems say Obama return from sidelines is overdue MORE (D-Mo.) won't join President Obama when he visits St. Louis on Tuesday, prompting claims by Republicans that she is avoiding an appearance with the president, whose poll numbers are underwater in Missouri.

An aide to McCaskill told The Hill that though she had hoped to be there, competing events scheduled for the senator and likely votes in Senate on Tuesday precluded her from returning to Missouri. The aide added that McCaskill hopes the timing will work out better in the future.

But Missouri Republicans have pointed to a more cynical reason for the first-term senator to stay in Washington: McCaskill's GOP opponents already have their video cameras fired up to capture footage of her appearing chummy with Obama. That footage will inevitably end up in campaign ads tying her to the president, in hopes that his reelection struggles might complicate her own.

McCaskill, a fiscally conservative Democrat who took her seat from a Republican in 2006 by a slim margin, is considered one of the most vulnerable senators up for reelection in 2012.

She was an early supporter of Obama during the 2008 Democratic presidential primary. Yet her chances for reelection hinge on her ability to distance herself from Obama and portray herself as an independent voice for Missouri who is willing to buck the national Democratic party.