Former Ohio Secretary of State Ken Blackwell (R) has endorsed former Rep. Rob Portman’s Senate bid.

Portman is the only announced GOP candidate in the race for retiring Sen. George Voinovich’s (R) seat, although state Auditor Mary Taylor is also looking at running.

“I have worked with Rob over the years, and I know he will be ready to lead on day one,— Blackwell said in a statement.

Portman also received his first interest group endorsement last week from the Associated Builders and Contractors of Ohio.

But while Portman appears to have almost cleared the GOP primary field, several Democrats are running for their party’s nod. Lt. Gov. Lee Fisher, Secretary of State Jennifer Brunner and state Rep. Tyronne Yates have announced they are running, and several other Democrats are considering a bid.

Polling has shown the Democratic primary is a wide-open contest. Regardless of who wins the nomination, the race is expected to be one of the top Senate contests in 2010.