A Democrat challenge to Gov. Patrick in 2010?

Gov. Deval Patrick could be in for a shock come election time as his chief Democratic rival Treasurer Tim Cahill refuses to rule out a primary challenge for the Corner Office.

Patrick told the Herald in August that he intended to seek a second four-year term in 2010 - and that Cahill had assured him that he would not run against the incumbent governor.

“He tells me he’s not going to do it if I’m here, and I’m planning to be here, so I assume I’m not going to have to deal with him,” Patrick said during that interview.

But yesterday, Cahill would not rule out challenging Patrick.

The treasurer said he would “most likely” run for governor if Patrick isn’t in the race, but wouldn’t shut the door on running against him either.

“I don’t want to say either way, because I honestly don’t know,” Cahill said at a meeting with Herald reporters and editors. “It would depend on the situation that the state faces. Obviously, you’d have to believe you could do a better job, but it’s a huge uphill battle to challenge someone from your own party.”

A spokesman for Patrick’s campaign declined to comment on Cahill’s remarks yesterday. The governor has repeatedly insisted he will not take a post with incoming President Obama’s administration and will instead seek re-election - and Cahill says he believes him.

Cahill said he’s less interested in running for U.S. Senate than he is in making a bid for governor. “I enjoy being the boss,” he said. “I’m more interested in what’s happening here in Massachusetts.”

UPDATE: More from the Globe and Michael Graham.

SECOND UPDATE: From the Globe, Herald and Holly Robichaud.