Another Republican candidate for the U.S. Senate announced his run this week, but observers on both sides of the aisle — and from Colorado to the nation’s capital — are still left wondering when the “real” candidate is going to get into the race to challenge Democrat Michael Bennet.

Former Parker Mayor Greg Lopez, a former director of the Small Business Administration in the state, lit up the chatter circuit briefly on Monday when he tossed his hat in the ring, joining El Paso County Commissioner Darryl Glenn and retired Air Force programmer Charlie Ehler, who have both been running for the Republican nomination for months.

While Lopez knows a thing or two about running for the Senate — he ran for a state Senate seat in 2000 but lost that race — insiders were speculating hard this week whether he’s serious about the race (he says he is) or is a place-holder for another candidate.

The list of Republicans who have passed on the chance to take on Bennet is impressively long. Just recently, U.S. Rep. Mike Coffman declined to leave his competitive 6th Congressional District seat to run against Bennet; his wife, Attorney General Cynthia Coffman, made official what was already common knowledge, that she had no plans to run for the U.S. Senate; Republican National Committeeman and former state Sen. Mike Kopp bowed out after giving some thought to a run; and state Sen. Ellen Roberts said she wasn’t interested, following a ferocious backlash from opponents after she floated the possibility to a Durango reporter.

As to who might run, liberal political blog Colorado Pols speculated this week that former Colorado Solicitor General Dan Domenico is being heavily courted to jump in, although our sources tell us he has no interest in running. And there’s always the proverbial “business” candidate who might be able to self-fund and cut a nonpolitical swath through the field, but so far that name hasn’t emerged.

But according to a seasoned Republican insider, who knows a thing or two about winning U.S. Senate campaigns, don’t count out Mike Coffman. “There is still an outside chance that Mike Coffman could pull a Cory Gardner and change his mind later this year and run,” our insider says, noting that Coffman is raising money aggressively and could fold his federal funds into a Senate race. Otherwise, some legislators are surely considering it. And, our source reminds us, it just wouldn’t be an election year without perennial candidate Tom Tancredo floating his name.