WASHINGTON — A former Wisconsin governor, Tommy Thompson, yesterday joined the crowded field of Republicans running for the White House in 2008 and proclaimed himself the “reliable conservative” in the race.

Mr. Thompson, who was health and human services secretary during President Bush’s first term, also said he is the only GOP candidate who has helped assemble both a state and federal budget.

Since announcing last year he was forming a presidential exploratory committee to raise money and gauge support, Mr. Thompson has lagged behind better-known rivals.

Mr. Thompson, 65, has focused his strategy on Iowa, which holds the nation’s first caucuses for presidential nominees. He has made weekly visits to the state and sought to make the case that it will take a candidate who can carry the Midwest to win the nomination. “Things are started to coalesce, and I feel very, very optimistic about my future,” Mr. Thompson said yesterday, despite his single-digit polling.

“I am the reliable conservative. My record shows that. All that people have to do is look at my record, and I am one individual that they can count on,” Mr. Thompson said. Discussing some campaign issues, he said:

• He would have “a completely different Iraq strategy” from the president’s. Mr. Thompson said he would “demand” that the Iraqi government vote as to whether it wanted the U.S. to remain in the country. If the answer were yes, “it immediately gives a degree of legitimacy.” If the answer were no, “We would get out, absolutely. It’s a duly elected government.”

• He would veto the war spending bills in Congress that have timelines for an American exit from Iraq. “This is an invitation to continue the kind of civil war that’s going on right now. I think it’s the worst mistake,” Mr. Thompson said.

• Attorney General Alberto Gonzales has made “terrible mistakes” in the handling of the fired federal prosecutors. “I would not have appointed Mr. Gonzales. I would have appointed somebody that was loyal to me,” Mr. Thompson said.