Silver Spring realtor John P. Hewitt, who Montgomery county Democrats twice passed over as a nominee for county executive, says that he will seek the nomination this year as a Republican.

Hewitt, known in the county as a conservative, pro-business Democrat, switched to the GOP two weeks ago, on the last day legally possible to change parties and still run in the primary.

He said over the weekend that he will announce his third campaign for county executive at a press conference this week. He said he will campaign against the administration of incumbent Democrat Charles W. Gilchrist and the history of bickering and infighting on the all-Democratic county council.

"I don't think the fact that I switched parties will be an issue," Hewitt said. "My philosophy has always been moderate-to-conservative." He added: "I felt that the Republican Party offered more of a philosophy that I believe in."

Hewitt becomes the third Republican candidate for the county executive office. His entry into the race could divide Republican conservatives, who have been looking for an alternative to Luiz R. Simmons, a liberal.

Simmons, a Silver Spring attorney and one-term member of the House of Delegates, is campaigning for the nomination as a moderate-to-liberal Republican in the tradition of Sen. Charles McC. Mathias, former Rep. Gilbert Gude, and former Rep. Newton Steers. In a county with a 2-to-1 Democratic advantage in voter registration, Simmons is presenting himself as the Republican who stands the best chance of being elected.

But Simmons angered some conservatives when he opened his campaign by criticizing President Reagan's policy of reducing the size of the federal work force. The party's conservative-controlled central committee has since passed a resolution reaffirming support for Reagan.

Joseph McGrath, a banker, former Army officer and Republican fund raiser, is seeking the nomination as the candidate with enough business experience to bring private-sector-style management to the county government. Using banker's jargon, McGrath refers to Gilchrist as "the county's chief executive officer" and calls the government "one of the biggest businesses in Montgomery County."

McGrath had been hoping to gain support among the county's business community and traditional Republicans, some of whom feel that Simmons is too liberal and has not spent enough time winning the support of the party regulars.

Hewitt sought and lost the endorsement of the Democratic Party convention in 1970, and lost the convention's endorsement again in 1974. The last time, he ran in the Democratic primary and lost in a three-way race. Hewitt supported Gilchrist in 1978, but has become a vocal critic of Gilchrist since Gilchrist failed to appoint him to the Washington Suburban Sanitary Commission.