DENVER — Colorado Secretary of State Scott Gessler, who is exploring a run for governor, has alerted prosecutors of 155 non-citizens his office suspects have illegally cast ballots.

Gessler’s office sent letters to 4,201 people last summer asking them to clarify their status since last summer.

Of those, 155 have voted in one or more past elections and did not reply to the letters.

Letters were sent to people who once showed proof of non-citizenship, such as a green card, when getting a driver’s license and then later appeared on voter rolls.

“Under Colorado law, only citizens can vote in our elections,” Gessler said in a press release from his office. “We tried to prevent illegal votes before the election, but now we have no choice but to refer these cases to law enforcement for investigation and prosecution.”

Gessler is also using a federal immigration database, but critics say it’s not error-proof.

Democrats have charged that Gessler’s efforts to curtail voter fraud are thinly veiled attempts to use his non-partisan office to achieve partisan ends, namely, lowering voter turnout among Hispanic voters and other traditionally Democratic-leaning constituencies.

Gessler has also been dogged by questions about his ethics after Democrats raised concerns about his attempt to continue working for his old law firm, which specializes in election law, early in his term; more recently, he decided to pay back $1,278 he spent from a discretionary fund to pay for expenses incurred attending the Republican National Convention last August in Tampa, Florida.

If he enters the governors race over the summer, he’ll be bringing all of that political baggage with him, according to political analyst Eric Sondermann.

“John Hickenlooper, in most cases, is a Teflon politician,” Sondermann said. “Scott Gessler strikes me as the opposite. He’s a Velcro politician: everything sticks.”